Table 1.

Potential determinants considered when developing exposure groups.

Type of determinants	General determinants	Discussion
Agent	Pathway into body	An agent may be inhaled, absorbed through the skin, remain on the skin or ingested. Each pathway depends on the physical state of the agent and the opportunity for contact and affects which determinants are most important.
	Pure chemical or mixture	Chemicals may be in the pure state or contained in a mixture. The vapor pressure of a chemical in a mixture is less than the chemical’s corresponding vapor pressure in the pure state and thus can result in lower exposure than would be observed when handling the chemical in the pure state.
	Composition of the mixture	The composition of a mixture generating a vapor may be static or it may be changing. If the relative composition is changing slowly enough over a designated time period, it may be possible to consider the composition as relatively constant, i.e. in a pseudo equilibrium state. In a mixture, when the more volatile components evaporate over time, the impact changes the mixture composition. Because the overall mass of the mixture decreases (due to the elimination of the volatile components), the composition (i.e. % by weight) of the less volatile components in the mixture may increase with time. Once the volatile components have evaporated to a significant degree, the change in composition of the mixture may be very slow. Depending on the component, a change in composition can result in either an increase (for the more volatile components) or decrease (for the less volatile components) in exposure.
	Vapor pressure of agent	An agent’s vapor pressure controls how much vapor of the chemical is generated and in turn, how much vapor is available to be inhaled by the worker. If the liquid/vapor concentration is in equilibrium, the vapor concentration is referred to as the saturated vapor concentration (SVC). In most actual exposure scenarios, the concentration of vapor in air that workers encounter is usually a small percentage (<<1%) of the SVC, with the actual percentage of the SVC dependent on a number of factors such as the
		effectiveness of ventilation (general and mechanical), position of the workers with respect to the source of emission, wind speed, etc. While the agent’s vapor pressure controls the maximum concentration of vapor in the air above the liquid, the evaporation rate (mass/unit time) is affected by additional factors including: the agent’s molecular weight, the size (surface area) of the pool of liquid, length and width of the pool and the velocity of the air moving over the pool. An increase or decrease in vapor pressure can increase or decrease exposure, respectively.
	Temperature of liquid generating the vapor	The vapor pressure of the chemical is affected by the temperature of the parent liquid generating the vapor (not by the ambient temperature of the air). An increase or decrease in temperature can increase or decrease exposure, respectively.
	Surface area	The surface area is the area of the liquid exposed to the air, and the greater the surface area the greater the amount of the agent that can be volatilized. The surface area may be due to either the liquid being in a pool on a surface or being in a container with an opening. Where the liquid is in a container, the potential exposure is dependent on the size of container’s opening rather than the quantity of liquid in the container. The term container is being used in a very general way. It could be a vessel, process equipment, storage tank, pail, bottle, tote or transfer line, and the opening could be a deliberate opening (i.e. without a cap) or inadvertent opening (a pipe leak). If an operator is adding an agent to a process, the opportunity for exposure to the vapor is proportional to the number of times the agent is added.
	Quantity	The quantity of an agent may be important if it increases or decreases the surface area available for evaporation; however, an increase in quantity may not affect the exposure level if the surface area does not change. The same surface area can have a large or small quantity depending on the configuration of the liquid’s container, i.e. a tall, wide container can have the same surface area as a short, thin container if the opening of the 2 containers is the same. In this case, quantity is not important (as long as some liquid is present).
Workplace	Emission points • Number Characteristics • Size • Emission rate • Mechanism of release	Emission points relate to positions where vapor is introduced into the workplace. There may multiple emission points in a workplace and increasing the emission points can increase exposure and vice versa. In an enclosed configuration, there is a zone of exposure in the worker’s immediate breathing area and a zone containing the remainder of the work area, such as a room. There will be an exchange of air between the zones, but the vapor near the emission point in an enclosed area is not diluted as quickly as would be expected in an open area with even light winds. Each point can have a differently sized surface area and a different emission rate of the vapor (the speed at which the emission is released into the atmosphere) that contribute to the exposure (a greater size of the source or a greater rate can result in increased exposure and vice versa). The type of release mechanism can affect the speed at which the agent evaporates (spraying results in faster evaporation, and therefore exposure, than no spraying), as can the direction of the release (a release with more energy results in a narrower plume than a release with less energy, which can increase or decrease exposure depending on the worker’s location in relation to the plume).
	Ventilation or engineering controls Types • General or natural dilution • Capture • Containment Characteristics • Height/location of vents • Efficiency • Location relative to the source	Ventilation provides a mechanism to remove or prevent vapor concentrations from entering the worker’s breathing zone and/or the workplace. General dilution occurs with the presence of ceiling or room fans. Natural dilution ventilation indoors may be due to open windows or doors. Both types of ventilation are generally ineffective at consistently and efficiently reducing vapor concentrations in the worker’s breathing zone if the worker is located near an emission point. Capture refers to collecting the vapor at the emission point (e.g., a duct with slot hood). With a capture configuration, the source of emission is outside of the duct, but the net flow of air into the duct is at a rate sufficient to capture molecules being emitted from the source. Depending on the configuration of the capture device, the efficiency of capture ventilation can vary significantly. Containment means that vapor is emitted to an area that is isolated from the worker (e.g., a closed process). There also is a variation of containment where capture ventilation is used within the contained area to further enhance efficiency (e.g., a duct with a slot hood located inside a contained area such as a lab hood). The height or location of a vent can facilitate movement of contaminated air to a different area. These characteristics may decrease exposure (by moving the air away from workers) or increase exposure (with improper design, such that the contaminated air is vented into an area with workers). Mechanical ventilation may be ineffective if improperly designed or maintained.
		Ventilation may have no effect on exposure if the ventilation is poorly located, such as being too far from the emission point to be effective. Random air speed (non-directional) indoors is on the order of 4 m/min (0.15 mph), and directional air speed due to air changes per hour (ACH) indoors is also very low. For example, consider an average size room (width, length and height of 4 m by 4 m by 3 m, respectively) with 6 ACH, which is considered good general ventilation. Although air flow patterns are very complicated, if very simple conditions are assumed where air is entering through a vent on one side of a room and exiting through a vent on the other side of the room, the directional air speed is equivalent to about 0.4 m/min or 0.02 mph. That is, indoor air speeds are low compared to even virtually still winds outdoors (see Wind, below).
	Wind • Speed • Direction	Air velocity increases the evaporation rate. A wind over the liquid can effectively carry vapor away from the workplace, reducing exposure, but it can raise exposures if so much air movement is present that it overwhelms the mechanical ventilation efficiency or if the movement contaminates areas downwind where the agent of interest would normally not be present.The intensity of an exposure is usually significantly higher indoors or in protected areas where the wind, if present, is relatively low. Wind can be directional or non-directional. Non-directional movement of air molecules from one position to another can dilute air near an emission point and raise air concentrations further from the emission point. Directional winds can increase the air concentration within the plume and lower it outside the plume, if clean. If contaminated, the opposite can occur. Evenly virtually still winds outdoors are 25 to 50 m/min (1 to 2 mph).
	Pressure differences in an area	The net pressure on the system can affect exposure. Contaminants flow from areas of higher pressure to lower pressure. Differences in pressure can occur during the filling or emptying of a container. For example, if a worker is monitoring the filing of a tank neat the opening to the tank, the displaced air from the tank (i.e. the flow of air out of the vessel) can increase exposure to the worker beyond that expected from diffusion. If a worker is monitoring a tank that is being drained from the bottom (i.e. air is flowing from outside of the tank into the tank), the expected exposure may be less than that observed with diffusion. The use of positive pressure in an area to prevent air contamination from entering into an area, such as in a “clean” room, is another example of how differential pressure can affect exposure.
Work Practices of activities/tasks	Location of the worker (relative to emission points, ventilation, and wind)	Distance from the emission point refers to the distance of the worker from the emission point. The distance influences how concentrated the vapor is when it reaches the worker due to interference by general, natural, or mechanical ventilation. Generally, the greater the distance, the lower the concentration and vice versa, as can be shown using a Gaussian Plume Model. For example, consider conditions of low wind <27 m/min (< 1mph) and a small surface area between 26 and 232 cm2 (0.03 to 0.25 ft2). If exposure at 0.3 m (~1 ft) from a source is 100 ppm, at roughly 0.75 m (~2.5 ft or arm’s length), the exposure drops to 18 ppm. At ~1.5 m (about 5 ft), the exposure drops to 5 ppm and at 3 m (about 10 ft), the exposure would be only 1.5 ppm. A worker’s exposure may be higher if the worker is located between an emission point and the exhaust point of the ventilation; alternatively, the exposure may be lower if the worker is outside the emission to ventilation plane. If a wind plume is moving clean air, a worker in the plume can have a lower exposure than if outside the plume; conversely, if the wind is moving contaminated air, exposures will be lower outside the plume than in the plume.
	Duration and frequency of an activity/task	Exposure can be affected by the task duration and the number of times (frequency) that the task is performed daily, weekly, etc. The longer the duration or the more frequent the exposed activity/task is performed, the higher the (cumulative) exposure and vice versa.
	Isolation	Isolation prevents workers from entering an area contaminated with the vapor, reducing or eliminating exposure.
	Other agents present	Specific activities/tasks can result in exposure to the agents of interest such as cleaning chemicals, fuels, etc. resulting in additional exposures beyond that attributed to the source.
Workforce	Personal protective equipment (PPE)	PPE can decrease exposure, but improper use of the equipment can result in an increase in exposure. Also, a false sense of protection when the PPE is not properly worn can result in behaviors that can increase exposure. It is important to know how long the PPE is worn and how frequent the PPE is replaced or decontaminated.
	Cleaning/ decontamination	Especially with dermal exposures, the decontamination or cleaning of the body part impacts exposure, i.e. the frequency hands are washed or a shower is taken. Additionally, the frequency that clothing is changed or PPE is decontaminated can impact dermal, inhalation and in some cases, ingestion of the agent. Contaminated clothing can result in continued exposure to an agent even after the task involving the agent is completed.
	Frequency and duration of tasks performed	The exposure frequency and duration can impact the effect on the body in addition to the exposure level. For example, the body may very efficiently eliminate or detoxify an exposure that occurs once a wk for a duration of 4 hr but if the exposure of the same intensity is encountered virtually every day, for the entire day, the capacity of the body to safely rid itself of the agent may be compromised.
	Shift	The activities performed, and their frequency and duration, may vary from shift to shift for a job title. For example, process samples may be collected during the night shift so they are available for analysis in the laboratory, which may be staffed only during the day. Activities like drumming product may occur during all shifts worked, but other activities such loading tank trucks may occur only during the day shift. Maintenance may be done 24 hrs/day, but the night shift may have different exposures due to performance of different maintenance activities or due to the shutdown of day production lines.
	Job rotation	Job rotation may affect exposure several ways. First, if a given job title works only one shift (days, evenings or nights), workers with the same job title within the same department may have very different exposures as described above in “Workforce/Shift”, depending on the shift. Second, it is common practice for a job title within a department, e.g. “operator”, to have multiple job assignments. For example, a process area may involve four job assignments: control room operation, raw materials addition, production, and product packaging, each being performed by workers with the job title of “operator”. In this example, although the job title is the same for all 4 assignments, the workers may have very different exposures because of the exposure differences of the assignments. It is not unusual, however, for an “operator” to perform all 4 of these assignments. If the rotation through these assignments is the same for all workers, all workers would have a comparable exposure. Alternatively, some workers may have different skill levels or levels of training. As a result, some workers may rotate through all 4 assignments while others may rotate through only 2 or 3 of the assignments. In contrast, in some cases,
		different job titles, e.g., “senior operator” and “operator”, may be assigned to workers even though the job assignments are identical, with the former title being a reward for more experience or longer tenure with the employer. Third, the rotation schedule for job assignments across the shifts (days, evenings or nights) may vary. For example, it the operation is continuous, four shifts (A, B, C & D (~40 hours each)) are required to cover the 168 hours in a week. Rotation among shifts or among job assignments may occur weekly, monthly or over a longer period of time, such as every 3 months. This rotation can impact the potential for an adverse heath outcome due to a lower recovery time, depending on the disease mechanism. Under the same workplace conditions, however, such varying rotations generally would have little impact on exposures. In contrast, in custom industries, such as machine shops or the pharmaceutical industry that run campaigns of varying duration for different products, different exposure levels to different agents could occur, because the job rotation is not synchronous with the campaign duration.