模塊1：職業衛生原則 (Module 1: Occupational Hygiene Principles)

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Hello. Welcome to "Occupational Hygiene Principles". My name is Pete Raynor.
I’m an Associate Professor at the University of Minnesota School of Public Health.
The learning objectives for this module are that, by the end, learners should be able to classify the types of hazards that workers face,
define exposure and related terms, list the routes by which workers can be exposed to hazardous agents,
and describe the occupational hygiene framework of anticipating, recognizing, evaluating, and controlling workplace hazards.
As we begin to discuss the occupational hygiene framework, I should make the point that "occupational hygiene" is a term used
interchangeably with "industrial hygiene". "Occupational hygiene" is used more frequently in Europe and other parts of the world
whereas "industrial hygiene" is used more commonly in the United States.
I'm primarily going to use "occupational hygiene" in this module because I think it describes the profession more accurately.
The American Board of Industrial Hygiene defines industrial hygiene as "the science of protecting and enhancing the health and safety
of people at work and in their communities”. This definition makes sense from the standpoint of protecting people at work.
However, it also includes another critical aspect of occupational hygiene:
the protection of people in the community who may be affected by what others do at work.
Goelzer defines occupational hygiene as "the science of the anticipation, recognition, evaluation,and control
of hazards arising in or from the workplace and which could impair the health and well-being of workers,
also taking into account the possible impact on the surrounding communities and the general environment."
This second definition of occupational or industrial hygiene is more comprehensive.
I also like that it includes the occupational hygiene framework of anticipating, recognizing, evaluating, and controlling hazards.
We need to be able to understand when potential hazards may be present, notice them when they are there,
know how to determine if they are a problem, and then do something about them if they are a problem.
With whom do occupational or industrial hygienists interact?
They interact with people on the job. Most important are the workers.
However, this also includes the owners, managers, and supervisors of different workplaces without whose support we can get
little accomplished, regulators like those that work for the Occupational Safety and Health Administration or OSHA
at either the federal or state level, and members of the public who may be affected by what goes on at the worksite.
Occupational hygienists interact with their colleagues in other health and safety fields including occupational physicians,
occupational health nurses, safety and environmental specialists, occupational epidemiologists,
and occupational hygiene technicians who may carry out some of the measurements and sampling that are designated by hygienists.
Finally, occupational hygienists also interact with engineers and facilities and maintenance personnel.
Engineers may be tasked with carrying out changes in workplaces to make them more healthy or safe,
and facilities and maintenance personnel can affect workplace health and safety by the regular maintenance they conduct,
the cleaning that they do, and other changes that they may make to work environments.
Let’s talk next about anticipating and recognizing hazards.
The way I'm going to do that, at least initially, is to talk about my high school job
I worked at a grocery store in Irondequoit, New York, a suburb of Rochester.
I worked there from late in my junior year of high school through the summer after my sophomore year in college.
When I went to work at the grocery store, my first job there was to collect carts outside.
Today, many cart pushers have automated, battery-powered pushers to move trains of carts.
Back when I did the job, I worked much like the person in this image: we assembled long trains of carts and pushed them across the parking lot.
So, there was a lot of a heavy pushing, and there was also a chance to get your fingers pinched between the carts.
It was hard, tiring work. We would sometimes make very long trains of carts on purpose to see how long we could make the train and still
push it back to the front of the lot. Sometimes, the long trains got a little out of control and hard to stop because they had so much momentum.
We had to be concerned about vehicles in the parking lot. Often, the parking lot was congested, particularly at busy times around
holidays or on weekends. Although I was never injured by an incident with a car, there were many close calls and I had to watch myself
around cars that were moving too fast or were having difficulty backing out of a parking space.
During different times of the year, we were exposed to either heat or to cold. Because Rochester, New York is on Lake Ontario,
we had a lot of lake effect snow and frequently it would be slippery in the parking lot during the winter in addition to it being cold.
In the summertime, it could be very hot and we would feel it after four hours of pushing carts around.
We would sometimes be faced with shoplifters when we worked outside the store.
On one particular occasion, I was collecting carts on a sunny afternoon.
I was out there by myself, it wasn’t very busy, and I was off in my own little world as I was picking up carts.
So, I wasn’t really paying too much attention to what was going on other than in my own immediate vicinity.
Suddenly, I saw this guy running toward me, and as he got closer, he yelled to me, "Don't do it, man! Don't do it!" I'm thinking to myself,
"Don't do what?" The guy ran past, and I was still trying to understand what he meant, when my supervisor runs up to me and says,
"Pete, why didn’t you stop him?" I thought to myself, "Stop what?" I had no idea what was going on. It turned out that he was a shoplifter.
So, there was a risk that, had I realized what was going on and tried to intervene, I could've been subjected to some violence.
My supervisor was a little disgusted with me for not divining that I should have stopped the guy.
This particular supervisor didn’t like me very much, and it was sometimes stressful to deal with him.
Eventually, I was given the opportunity to move inside, at least most of the time, and be a checkout clerk, which involved scanning a lot of
items, putting them into bags, and lifting the bags up and over a shelf to put them into the customers’ shopping carts.
I would also have to enter numbers into the keypad on the register. Shifts on the registers could last as long as eight hours with
a lunch and two short breaks. Standing in one place and repeating these same actions over and over was tiring.
One of the other things I would sometimes do when I worked inside was to go to the
back room of the store and bring sacks of paper bags – 500 per sack – up to the front end.
We didn’t have plastic bags back then, so these were thick paper bags.
I would take a large cart to the back room, go into the truck trailer where the sacks were stored,
climb a pile of sacks, toss about 15 sacks from the pile onto the bed of the trailer,
pick the sacks up and pile them onto the cart, and push the heavy cart through the store to the front.
Once there, I would have to unload sacks by each register, open the sacks with a box cutter, and stack the bags in the storage area
below the register where the clerks could pull them out and use them for customers. It was a pretty tough job.
When I worked inside at the front end, there was always a chance that we would have to deal with those shoplifters.
One time, a couple of us chased after some shoplifters who were trying to steal beer late at night.
We got out into the parking lot and found that we were a little outnumbered because the shoplifters were part of a larger group.
Fortunately, the shoplifters and their friends decided to leave the beer on the ground and drive away rather than forcing a confrontation.
Another time, several of us were working up front, and a car pulled up outside the front window of the store right near us.
A bunch of guys piled out of the vehicle after popping the hood, and we could see that there was a fire in the engine compartment.
We were, naturally, concerned about this, as you might expect. After a frantic few seconds discussing what we should do,
I ran out of the store with our fire extinguisher and put out the fire, trying to keep as far away as I could.
We were surprised about 10 minutes later when the guys piled back into the car and drove away.
I'm not sure how far they got; it was a topic of discussion for my co-workers and me for the rest of the evening.
Among some of the other duties I had was occasional maintenance work, especially on weekends.
There was one time on the Monday of Labor Day weekend that I had to clean both the men's and women's bathrooms for the store.
It was clear that they had not been cleaned since the previous Friday, and it was a pretty eye-opening experience
to have to clean those restrooms after that amount of time. In short, it was not a fun job.
There was lots of nasty stuff in the bathrooms, and a variety of cleaning products needed to be used.
Another one of my occasional tasks was to take materials to the back room to be disposed of.
Large groups of fluorescent bulbs are changed out at the same time in stores to make the task easier logistically.
This was the early 1980s: the fluorescent light bulbs were not recycled at that time.
When I was asked to dispose of the bulbs that had been changed out, I would take barrels full of them back to the trash compactor in the
back room, pile them into the trash compactor, close the door to the compactor room, press the button to turn the compactor on,
and we would then hear the bulbs shatter in the compactor. As a teenager, this was a pretty cool thing to hear all the crashing and smashing
and eventually, after the compactor had stopped, we would open the door,
and we could see an almost magical haze of shiny glass particles floating in the compactor room. It was awesome to look at, man!
Probably the most fun I had on the job was on one full-day shift on the Saturday after Thanksgiving when my co-worker Todd and I
had the opportunity to hang holiday decorations for the entire day. We climbed ladders and reached out to hang things across the ceiling,
We climbed ladders outdoors and hung garlands and decorations across the front of the store. There was a lot of climbing up and down and
reaching this way and that, but it was a really fun day because it was an unusual task to get paid to do. We enjoyed it quite a bit.
By this point, you may be wondering, “Why is this guy droning on about his high school job?”
Well, let’s think about my job and about the potential hazards that I faced on the job.
There were hazards that could have caused unintentional injuries: fires like from the car, vehicles in the parking lot,
slips in the parking lot when I pushed carts, falls from ladders when I hung holiday decorations, sharp objects like box cutters,
and pinch points like when the carts come together and you pinch your fingers as you try to line them up.
In addition to the unintentional injuries, there was a risk of intentional injuries. Violence was a risk particularly when I faced shoplifters.
Repetitive motion injuries were quite possible. I faced the risk of an injury to my back from pushing carts,
lifting sacks of bags in the back room, and lifting full bags into customers' carts at checkout.
Wrist injuries were possible from continually scanning items at the checkouts for long periods of time.
Temperature extremes when I worked outdoors, both when it was hot and cold, could have led to heat or cold strain.
I was exposed to germs at work. Because I was dealing with members of the public,
I could've been exposed to their germs as they sneezed or coughed near me and when I handled their money.
There were also the times when I cleaned the public restrooms when there was a potential for exposure to germs.
Chemical exposures from cleaning chemicals and the mercury in the fluorescent bulbs could have been a health concern, too.
Even stress could have been a concern. I knew that my supervisor didn't like me very much.
If I had cared more about the job than I did, I may have felt stress that could have impacted me negatively.
There was a whole range of different hazards that I was potentially exposed to in my workplace.
Although you may have been able to anticipate that grocery workers face workplace hazards, most of you, unless you've worked
a very similar job, would not have been able to recognize all of these different hazards.
This is an important point because when you are trying to anticipate and recognize hazards,
you really need to get to know the job before you can be effective at analyzing the hazards.
Ultimately, workers are the experts on their own jobs. If you are trying to understand where there
is a potential for hazardous exposures to whatever sort of agent you're concerned about, you need to talk to the workers.
If we generalize and categorize some of the hazards that exist in different workplaces,
we can anticipate and recognize chemical hazards that include airborne particles such as nanoparticles.
Workplaces may contain different gases and vapors, and especially solvent vapors as many, many solvents are used in industrial settings.
Heavy metals may be present, including molten metals, metals used in electronics production, and metals released to the air during machining.
A large variety of skin irritants may be present as well, with dermatitis being one of the most common workplace diseases.
There are physical hazards, hazards that affect the senses or the whole body.
These include noise, ionizing and non-ionizing radiation, and hot and cold temperature extremes.
We can anticipate and recognize biological hazards, including infectious disease agents,
which can be of particular concern in the healthcare industry, and mold.
Agriculture workers may face mold in outdoor environments or in barns,
and construction workers may be exposed to mold during renovations.
Offices that have experienced water damage may see mold growth in walls, ceilings, and carpeting.
There are injury hazards. Unintentional traumatic injuries can occur.
These include vehicle crashes, which are one of the most common causes of fatalities on the job.
Violence, either among co-workers or involving both workers and people from outside the workplace, is an important occupational hazard.
In addition, poor ergonomic conditions, including repetitive motion, awkward posture, and heavy lifting, may lead to musculoskeletal disorders.
Occupational hygienists may be able to anticipate, but find it hard to recognize,
social and behavioral hazards like stress, sleep deprivation, and substance abuse.
These hazards can make it difficult for a worker to perform her or his job safely
and in a healthy manner, in addition to being risk factors on their own.
and in a healthy manner, in addition to being risk factors on their own.
Let's move on and talk about evaluating hazards. Why would we want to evaluate hazards? I list six purposes here.
Let's move on and talk about evaluating hazards. Why would we want to evaluate hazards? I list six purposes here.
First, we might evaluate hazards for compliance purposes. The goal is to compares workers' exposures to an exposure limit or a standard.
For example, we can compare sound levels in a metal stamping operation to the
Occupational Safety and Health Administration's permissible exposure limit for noise.
We might try to measure levels of a hazardous agent throughout a work environment with a goal of identifying the source or sources of the agent.
An example would be to create a concentration map, almost like a topographical map,
of a machining facility to identify the sources for emissions of oil mist.
In emergency situations, we might seek to detect hazards that are immediately dangerous to life and health.
An example is the need to monitor hydrogen sulfide levels when workers enter a manure pit to perform maintenance or cleaning.
Control measures might need to be evaluated. The goal in this case would be to ensure that interventions designed to reduce hazardous
exposures are working as planned. An example of this is a series of measurements performed to ensure that airborne particles containing mouse
urine proteins are kept within ventilated enclosures during the change out of research animal cages in university settings.
We might also evaluate hazards as part of research.
The goal will depend on the hypothesis that is being investigated, which is sometimes part of a larger occupational epidemiology study.
An example of this that I worked on was when we measured silica dust concentrations as part of an epidemiological study
to determine the effect of the dust on the lung health of taconite ore miners on the Iron Range in northern Minnesota.
Finally, we might evaluate hazards for risk assessment purposes.
In a sense, all of these other purposes are forms of informal risk assessments. Here, however, I'm talking about a formal risk assessment
where the goal is to calculate exposure and/or dose for a worker exposed to an agent of concern, so that we might compare
that exposure or dose to the potential health effects from the dose in order to characterize the risk of an adverse health outcome.
An example is to measure radon concentrations in building subbasements to estimate cumulative doses that workers receive.
We will talk more about risk assessment and risk characterization in the next module.
How do we go about evaluating hazards? One way is by measuring them. We may measure a hazard to detect it, just to see if it is there or not
or we may want to know its concentration in a medium like food, water, or, especially in occupational settings, air.
In addition to measuring agents in the environment, we can measure things called biomarkers within exposed people.
In our context, biomarkers are substances measured in some part of the body that indicate the presence of an agent in the body.
A biomarker may include a chemical of concern or its metabolite, or some other biologic indicator of exposure.
This "biomonitoring" can be performed on samples of urine or blood or you can measure substances in tissues or hair samples.
We can attempt to evaluate hazards using modeling. We are not able measure everything everywhere at all times.
One way to get around these limitations is to use mathematical models to estimate exposures. The models can be used to predict
concentrations or other relevant measures of exposure as a function, ideally, of both time and location.
Ultimately, we will compare these measurements or modeling predictions to some sort of occupational exposure limit.
These occupational exposure limits are developed through the risk assessment process,
which, as I mentioned previously, will be discussed in a future module.
In short, we relate health risk information from toxicological and epidemiological studies to exposure or dose data,
decide what is an acceptable risk, and set an exposure limit accordingly.
When making measurements after performing the risk assessment,
we can compare our findings to the exposure limit to determine whether the workplace may be unduly harmful to people.
We can start to discuss controlling hazards by taking a look at some relevant definitions.
The vocabulary used may vary depending on one's perspective.
We can think about "managing" hazards. Merriam-Webster defines "manage" as "to work upon, or try to alter for a purpose".
The term "limit" is defined as "to curtail or reduce in quantity or extent".
To "intervene" is "to come in or between by way of hindrance or modification".
Finally, to "control" is "to reduce the incidence or severity of, especially to innocuous levels".
In many cases, these four words are used interchangeably when talking about ways to reduce exposures.
The default word is often "control", but "control" doesn't appeal to some experts because it implies that you are always able to make the
changes that you would like to in order to reduce exposures to hazards. The term also lends itself better to technological approaches to
reducing hazard levels, whereas words like "manage", "limit", and "intervene" seem to leave open a broader array of approaches.
In the way that I use the term "control", I intend to leave open a wide variety
of means for reducing exposures, not only technological options.
We are going to use the word "control" going forward in this module, but keep in mind that the other words convey equally valuable concepts.
We refer to a "hierarchy of control" when discussing approaches to reducing hazard levels.
The hierarchy goes from most preferred at the top to least preferred at the bottom. At the top is elimination of the hazardous agent.
Can you just completely get the hazard or the process that generates it out of the workplace so that it's not an issue anymore?
That option is rarely viable because it would involve removing a process or a product that is essential at that place of work.
Next on the hierarchy are engineering controls, which are physical, chemical, or biological changes made to a process or a product
that reduce exposures to the hazard. Third on the hierarchy are work practice and administrative controls, which are changes in
how, when, or by whom tasks are performed in order to reduce exposures.
At the bottom is personal protective equipment, or PPE, devices and garments worn by workers to protect themselves from injury or illness.
Let's talk a little more about why these approaches are placed in the hierarchy in the order that they are.
Elimination is at the top because it is completely effective for all workers,
and because the responsibility for change is not placed on the exposed person.
Engineering controls are second on the hierarchy because, even though they don't completely eliminate the hazard,
measures are put in place that should reduce exposures for everyone,
and the responsibility is not placed on individual workers to reduce their own exposures.
Engineering control includes a variety of concepts such as substituting one type of material for another in a process or a
product, using automation so workers do not need to be as close to a hazard, isolating the person from the process generating the hazard
or isolating the process from the person, drawing contaminated air from a process away from workers using ventilation,
and installing control equipment such as an air filtration unit to separate a hazardous agent from the medium in which it is embedded.
Work practice and administrative controls are lower on the hierarchy
because both management and exposed workers are responsible for making changes.
Therefore, we are starting to rely on people to always perform their work in a certain way or at a certain time, which may be difficult to achieve.
Finally, personal protective equipment is the least preferred approach when other options
are feasible because individual workers must use the PPE correctly each and every time they
perform the task that creates the hazardous exposure in order to be sure that they are not exposed to the hazard.
Ventilation is an important concept that we should spend a little extra time talking about. In particular, I will focus on local exhaust ventilation
systems, which are intended to draw contaminated air away from close to the point of generation before workers can be exposed.
There are many different types of local exhaust ventilation.
These types include exterior hoods. Both images in the upper left show a flanged opening connected to a flexible duct.
A fan or blower downstream draws contaminated air into the opening during the
cleaning of a chamber in which nanoparticles have been produced.
Because the opening, in essence, needs to reach out and bring in the contaminated air, it is an exterior hood.
Another type of exterior is shown in the other image. This is a slotted hood where air is drawn through slot openings,
taking away particles and vapors that are produced in front of the slots.
This is an exterior hood because the hazardous pollutants are outside the hood and must be drawn into it.
Another category of local exhaust ventilation is partial enclosures.
An example of a partial enclosure is the laboratory hood on the lower left, in which any process generating pollutants is enclosed
on at least five sides and the pollutants are drawn away with the air flowing into the hood.
This air enters the hood through the sixth side where workers can access the process.
On the right is a drawing of a bagging process. There is a clamp over the opening to the bag from which, ideally, little dust will escape.
To ensure that any dust that does escape does not present an inhalation exposure risk to workers,
there is a partial enclosure around the bag filling area that is ventilated and will draw away any dust that escapes through the clamp.
Partial enclosures combine the concepts of isolation and ventilation, but they are not complete enclosures.
A ventilated glove box can be thought as virtually a complete enclosure. Workers can put containers of potentially toxic materials
inside the glove box, use the gloves to open the containers, manipulate the materials with any
releases into the air being drawn away by the ventilation, close the containers again, and then remove the containers safely from the glove box.
Let's consider work practice and administrative controls next. Work practice controls alter how workers perform a task.
Ways in which someone can do work differently might include scooping powders rather than pouring them from containers in order to reduce
airborne dust exposures, regular maintenance of equipment, regular cleaning of work surfaces, using wet cleaning instead of dry methods so
that not as much dust is produced, washing hands properly to prevent exposures to agents on hands when workers eat or go home,
continuing education and training on how to work safely and in a healthy manner, and emergency drills so that workers know
how to exit their workplace while making sure to shut down critical processes that pose risks to responders on the way out.
Administrative controls are measures that change when and by whom work processes are conducted.
Examples include restricting access to areas with potential hazards so that fewer people will be exposed,
the use of hot, warm, and cold zones during the response to a hazardous materials spill where only a few people with high levels of
personal protective equipment are allowed into the hot zone, security procedures to ensure that
only people who are supposed to be at a work site are present, limiting work time to reduce mistakes due to sleep deprivation,
and scheduling potentially hazardous work operations during shifts when fewer workers are present.
On its web site, OSHA states the following about personal protective equipment:
"When exposure to hazards cannot be engineered completely out of normal operations or maintenance work,
and when safe work practices and other forms of administrative controls cannot provide sufficient additional protection, a supplementary method
of control is the use of protective clothing or equipment. This is collectively called personal protective equipment or PPE."
The kinds of PPE that people wear to protect various parts of the body range from hard hats and safety glasses to hearing protection,
respirators, gloves, safety shoes, and protective clothing. There are many different types of PPE within each of these classifications.
The different kinds of protection need to work together and they can be very effective, but they require workers to use them properly.
A worker wearing personal protective equipment must be attentive every single time she puts it on and takes it off.
Otherwise, the PPE may not protect effectively against harmful exposures.
It’s challenging to be consistent with the use of PPE so that the protection can also be consistent.
Let’s introduce some general concepts regarding exposure and dose.
Exposure can be defined generically as "the intensity of the agent in question, time-averaged in some way relevant
to the adverse health outcome, at an appropriate interface between the environment and the population or individual at risk".
I have underlined and highlighted three sets of words, starting with "intensity".
Part of exposure is the amount of some potentially-hazardous agent. I’ve also highlighted "time-averaged", as there is also
a time component to exposure. Because exposure takes into account both quantity and duration, a higher exposure can occur if
either a greater amount of an agent is present or if a worker is in the presence of the agent for a longer time. They are both important.
I have also highlighted "appropriate interface" because exposure must be measured at the interface of the person with the surrounding
environment. The best interface for an airborne exposure might be if you could put some sort of sampler directly in front of a worker’s nostrils.
However, that's not very practical. Instead, we often try to locate a sampler in what we call the breathing zone of a worker
by hanging the sampler on a worker’s collar so that it is close to where the worker breathes, without interfering with the breathing.
A generic definition of dose is "the cumulative amount of a property derived from an exposure
that drives a biological response within the exposed organism".
I have highlighted the words "cumulative amount" because a dose accrues over time as a worker is repeatedly exposed.
I’ve also highlighted the words "within the exposed organism". This is important.
While an exposure to an agent is outside the person at the interface of the person with the environment, dose is what gets inside.
This is the main difference between exposure and dose.
This diagram is from a paper by Sexton and co- authors. It shows the environmental health paradigm.
We think about agents being emitted into the atmosphere of a workplace environment. There is a source that emits the agent into the air or
another medium, and there are pathways by which the emitted agent moves through the workplace before it reaches a person.
The agent is present at some sort of concentration, referred to as an exposure concentration, in the vicinity of the worker.
If the exposure is through the air, we can measure the exposure concentration
by sampling the air at an interface of the worker with the environment.
The agent can be inhaled or ingested or move through the skin to form a potential dose inside the body.
Some of the agent that gets inside the body may not be available for uptake, or some may be immediately removed
such as when you exhale a portion of a pollutant that you have just inhaled.
So, while this portion of the agent may be part of a potential dose, it is not being applied to the body. The portion that is applied to the body
– for example, the fraction of incoming particles that deposit in the lungs – is referred to as the applied dose.
Some of the applied dose may not be absorbed into the body; it may be excreted instead.
That portion which is absorbed is referred to as the internal dose.
From the internal dose, we move more into the realm of toxicology where materials may be delivered to certain organs or organ systems,
they may or may not have a biological effect after delivery, and those effects may or may not be adverse.
During the rest of the module, we will focus on how to calculate exposure concentrations, potential doses, and internal doses.
From Sexton and coauthors, a more formal definition of "exposure" is "contact of a biologic, chemical, or physical agent
with the outer part of the human body, such as the skin, mouth, or nostrils". This is similar to the definition that we looked at previously.
"Exposure concentration" is "the concentration of an environmental agent in the carrier medium at the point of contact with the body".
It’s an intensity or a quantity, as we’ve seen before, at the interface of the environment with the body.
"Potential dose" is "the amount of the agent that is actually ingested, inhaled, or applied to the skin."
And the "internal dose" is "the amount of the agent absorbed,
and therefore available to undergo metabolism, transport, storage, or elimination".
When we talk about exposure intensity, there are various metrics that we can consider.
The best metrics are ones relevant to the health outcome associated with exposure to the agent being investigated.
We can consider metrics like mass concentration, for example the milligrams of some substance per cubic meter of air.
Mass concentration is a common metric for measuring exposure concentrations of gases, vapors, and airborne particles.
For number concentrations, the number of microorganisms per unit volume or unit mass of air, water, or food is a common example.
Another example is the measurement of airborne fibers, when air samples are analyzed for the number of fibers per volume of air.
Intensity is measured for sound pressure levels using decibels. This is the basic unit for measuring exposures to noise.
Different units of concentration are used for different media. In water, we use units such as parts per million, parts per billion, or parts per
trillion. These units refer to the mass of the potentially-hazardous agent per mass of water. Taking into account the density of water,
it can be shown that one part per million of a substance in water is equal to one milligram of the substance per liter of water.
Similarly, one part per billion is equal to a microgram per liter, and one part per trillion is equal to one nanogram per liter.
For air, it’s different. For gases and vapors, we use units of parts per million or parts per billion,
but in air these units are considered to be on a mole-per-mole basis. While these units sound the same as the ones for water, they are not.
An entirely different conversion is required to convert from a measurement of mass per volume of air to parts per million or parts per billion in air
This conversion is based on the density of air rather than the density of water.
For airborne particles, mass per volume is preferred with units such a milligrams per cubic meter or micrograms per cubic meter.
For dose, we can calculate both dose and dose rate. Dose is typically specified in mass units for chemical doses:
the mass of a chemical taken in by a body during a specified time interval.
Dose rate is usually specified in units of mass per time for chemical doses.
Milligrams per day would be an example for dose rate whereas dose would just be milligrams.
Dose and dose rate can be normalized to an individual’s body weight, for example, milligrams per kilogram
of body weight for a mass-normalized dose or milligrams per kilogram per day for a mass- normalized dose rate.
There are a variety of routes of exposure. The three most common ones that we consider are inhalation (breathing in),
ingestion (swallowing water or food), and dermal exposures. However, there can also be exposures through the eye,
referred to as ocular exposures, auditory exposures through the ear, and whole body exposures to hazards like vibration or radiation.
Let’s discuss exposure and dose quantitatively.
We can define cumulative exposure over a certain time interval mathematically using this formula.
Cumulative exposure, E, is equal to the concentration as a function of time multiplied by the differential of time, dt,
integrated over an interval of time from t1 to t2. An integral is a calculation of an area under a curve.
If we think about a curve of concentration as a function of time and then calculate the area under that curve between two times of interest,
that is the same as performing an integration and would give you a cumulative exposure.
Average exposure concentration, c-bar, can be determined by taking the formula for cumulative exposure,
concentration times dt integrated from t1 to t2, and then dividing it by dt integrated from t1 to t2.
That is equal to the cumulative exposure, E, divided by delta t, the duration of the exposure, t2 minus t1.
Rearranging terms shows that the cumulative exposure is equal to the average concentration times the duration of exposure.
This is a good point, while we're thinking about exposure being equal to concentration multiplied by a duration of time,
to think about acute versus chronic exposures. Exposure is influenced by both quantity and duration.
Acute exposures to hazards typically have large quantities and short durations,
whereas chronic exposures typically have small quantities and long durations.
There are many types of exposures that are acute and many types that are chronic.
Sometimes, the same agent could be both an acute and a chronic hazard.
For instance, smoke from a structure fire can be an acute exposure that can produce very damaging health effects like smoke inhalation
over a short period of time. However, smoke from wood stoves, backyard fires, and other sources
to which people are exposed may not create an immediate health effect, but with chronic exposure over a period of many years,
those exposed may experience adverse health outcomes including respiratory diseases and even cancer.
Because both acute and chronic exposures can be important, when and how long we measure exposure matters a lot.
This is a plot of concentration on the vertical axis versus time on the horizontal axis. If the time interval we’re concerned about
lasts from time t0 on the horizontal axis to time t0 plus capital T, we might consider that to be most of a workday.
The plot of concentration versus time changes a great deal as a function of time over this workday.
The area under the curve, the gray area, is the cumulative exposure, the integral of concentration multiplied by time.
The horizontal line drawn across the gray area that is labeled c-average
is the average exposure concentration over the time interval from t0 to t0 plus capital T.
The area of the rectangle created by C-average has the same area as the gray area under the original curve.
That's all an average is: it’s the value that creates a box that possesses the same area as the area under the original curve.
The gray area is an example of the type of measurement we would make to assess a chronic exposure.
For these exposures, we often take measurements that last an entire workday.
However, if we're more interested in an acute exposure, the peak exposure that might cause an acute health hazard,
we might need to make a very short term measurement that would pick up this peak concentration
and compare that short-term exposure concentration to the risk from that exposure.
So, the duration of exposure may be very important depending on the type of health outcome we're interested in.
Moving on, potential dose is equal to the integral from time t1 to t2 of concentration multiplied by
a term we will refer to as the instantaneous contact rate multiplied by the differential of time.
The contact rate for inhalation exposures is the inhalation rate or breathing rate that may be in units of liters per minute, for example.
For food or water, contact rate might be the mass or volume of food or water consumed per day. It is how much of the medium –
air, water, or food – that the exposed person takes in per unit of time. In occupational hygiene, we frequently set potential dose
equal to the average concentration times an average breathing rate times the duration of time exposed.
Potential dose rate is dose divided by the integral of time.
This is equal to the average concentration times the average breathing rate times the exposure duration divided by the total time elapsed.
The elapsed time may or may not be equal to the time exposed. We will look at an example of this in a little while.
We will not spend much time with applied dose and dose rate.
We can calculate them from potential dose and potential dose rate by applying a unitless availability factor
that accounts for some of the potentially- hazardous agent brought into the body not being made available.
Examples include some portion of an agent that may not be able to access the skin for a dermal exposure
or particles that are inhaled but then immediately exhaled before they can come in contact with the lining of the lungs.
We frequently assume that the availability factor, alpha, is equal to one, which means that the applied dose and dose rate
are equal to the potential dose and dose rate.
This is certainly not always true, but we will consider it to be true for the rest of this module.
The internal dose rate, then, is the availability factor times the integral from t1 to t2 of the concentration,
multiplied by the contact rate, times a term we’ll refer to as the absorption factor, multiplied by the differential of time.
The absorption factor is the fraction of the agent that is brought into the body that is actually absorbed into the body.
Some of the agent may be eliminated before it can be absorbed, or it may pass through the body without being absorbed.
Frequently, we say that the internal dose is equal to the potential dose times an average absorption factor.
Similarly, we set the internal dose rate equal to the potential dose rate times the absorption factor.
This assumes, again, that the availability factor is equal to one.
Any of these doses or dose rates can be normalized by body mass.
We take the potential dose or dose rate, or the internal dose and dose rate,
and divide by the body mass to get mass- normalized doses and dose rates.
Let’s look at a couple of examples of these calculations.
In this first example, we will say that there is a Pesticide X that is released into a room at a kennel to kill fleas on a regular basis and is,
therefore, in the air all the time. We want to know the inhalation exposure to Pesticide X for a worker in that room.
We’ll assume that the worker weighs 50 kg and that Pesticide X is absorbed through the lungs at an average rate of 75%.
We've made a measurement that indicates that the air in the room contains a constant
Pesticide X concentration of 0.1 milligram per cubic meter.
The worker will spend 8 hours per day in the room and she breathes at a rate 18 cubic meters of air per day while she is working.
The two questions are, "What is the exposure concentration?"
and "What are the potential and internal and mass-normalized potential and internal dose rates to Pesticide X for this worker?"
So, what is the exposure concentration? Well, we’re given the exposure concentration in the problem statement!
The average concentration is equal to 0.1 milligram per cubic meter. We can multiply that by 1,000 micrograms per milligram
and say that the exposure concentration is also 100 micrograms per cubic meter.
For the second part, what are the potential and internal and mass-normalized potential
and internal dose rates of the pesticide for the worker?
Potential dose rate is equal to the average concentration times the breathing rate times the duration of exposure divided by the elapsed time
We know that the average concentration is 100 micrograms per cubic meter. We're given a breathing rate of 18 cubic meters per day.
The duration of exposure during each workday is 8 hours. The elapsed time, in this case, is one day.
We need to multiply one day in the denominator by 24 hours per day to be able to cancel units.
If we perform the calculation, we get a potential dose rate of 600 micrograms per day, on each workday.
The internal dose rate is equal to the potential dose rate times the absorption factor, which is 75% or 0.75.
So, we take 600 micrograms per day, multiply it by 0.75, and we get 450 micrograms per day.
The mass-normalized potential dose rate is equal to the potential dose rate divided by the body mass.
That is 600 micrograms per day divided by 50 kilograms, which equals 12 micrograms per kilogram per day.
Finally, the mass-normalized internal dose rate is equal to the internal dose rate divided by mass, which equals 450 micrograms
per day divided by 50 kilograms, which equals 9 micrograms per kilogram per day.
In a second example, we will consider a worker in a facility that produces polymer fibers.
This worker is exposed to an average of 7.9 milligrams per cubic meter of acrylonitrile vapor on the job.
He works 240 days per year, weighs 190 pounds, and breathes at a rate of about 22 liters per minute during his 8-hour workday.
The literature suggests that there is an absorption factor of 52% for acrylonitrile when it's inhaled.
The first question posed is, "What is your estimate of the worker's cumulative exposure to acrylonitrile during his 8-hour workday?"
The cumulative exposure can be calculated as being equal to the exposure concentration times the duration of exposure.
That is 7.9 milligrams per cubic meter multiplied by 8 hours, which yields a cumulative exposure of 63 milligram-hours per cubic meter.
The units for exposure may sound a little odd, but they take into account both the concentration and time elements of exposure.
For part (b), the question is, "What is your estimate of the annual internal dose rate
in milligrams per kilogram per year for the worker when exposed to acrylonitrile via inhalation?"
The internal dose rate is equal to the potential dose rate times the absorption factor divided by the body mass.
That is equal to the average concentration times the breathing rate multiplied by the time
exposed and the absorption factor divided by the time elapsed as well as the body mass.
Substituting values for the variables gives 7.9 milligrams per cubic meter multiplied by 22 liters per minute.
The time exposed is 240 days during each one year, which is the elapsed time, multiplied by 8 hours per day.
The absorption factor's 0.52, and the body mass of 190 pounds. Units conversions are needed,
so we divide by 1,000 liters per cubic meter, multiply by 60 minutes per hour, and also by 2.2 pounds per kilogram.
When we carry out the calculation, we get a mass-normalized internal dose rate of 120 milligrams per kilogram per year.
To summarize, occupational hygiene is the science, and to some extent the art,
of anticipating, recognizing, evaluating, and controlling workplace hazards.
This is also referred to as the occupational hygiene framework. Workers face a variety of chemical, physical, biological, injury,
and social/behavioral hazards at work, and workers are the experts on their own work environment.
We need to talk to them if we are going to try to understand the hazards that they face.
Many hazards can be evaluated using measurements or modeling, and by comparison to occupational exposure limits.
Options for managing those exposures or controlling them are selected based on a
hierarchy in which options that place the least burden on individual workers are preferred.
Exposure can be defined as the amount or intensity of an agent at the interface between a
person and his environment over a certain time interval, and dose is the amount of the agent brought into a person over a time interval.
This lesson was created by the Midwest Emerging Technologies Public Health and
Safety Training Program, or METPHAST Program, which is a collaboration among the
University of Minnesota School of Public Health, the University of Iowa College of Public Health, and Dakota County Technical College.
The METPHAST Program is funded by the National Institute of Environmental Health Sciences.
This module's content is solely the responsibility of its developers and does not necessarily
represent the official views of the National Institutes of Health.
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