Table 2.

Major potential determinants contributing to the study determinants.

Study determinant	Determinant considered	Discussion with examples used for GuLF Study EGs or exposure estimation*
Agents: THC, BTEX-H, dispersant aerosols and vapors, PM2.5, oil mist	Pathway into body Pure chemical or mixture	Both inhalation and dermal exposures were of interest. All agents of interest are mixtures.
	Composition (weathering)	See Supplemental Materials (SM) for a discussion of the weathering process due to evaporation and solubility. Supplementary Table S4 in the document presents each of the BTEX-H chemicals’ solubility in water; Supplementary Table S5 presents the impact of weathering on the composition of the crude oil and the make-up of the THC concentration. Finally, Supplementary Table S6 presents the amount of time it takes to reach a level of weathering, expressed in percent. Weathering therefore affected THC and BTEX-H, but it did not affect the other agents of interest (dispersant vapors, PM2.5or oil mist). The degree of weathering is expressed in percent (see Supplementary Table S5). The MC252 crude oil released during the DWH event contained 16.84% volatiles (by wt.). This 16.84 % included 1.68 wt % BTEX chemicals and about 1.4% n-hexane, i.e. for a combined ~20% of the volatiles. The SVC above the liquid crude oil expressed as a THC concentration (i.e. percent by volume) contained 5.58% BTEX vapor and 25.72% n-hexane vapor. That is, if the THC was measured to be 100 ppm, the combined BTEX concentration would be 5.6 ppm and the n-hexane 25.7 ppm. By the time the weathering reached 20% (2 to 3 hours, Supplementary Table S6), the BTEX chemicals increased to comprise 8.02% of the THC concentration and the n-hexane contribution was reduced to 21.05%. Weathering reached about 40% weathering within 2 to 3 weeks after the oil release, with the THC concentration comprising about 12.86% BTEX vapor and the n-hexane vapor concentration reduced to about 4.33%.
		The change in weathering was used to identify areas in the Gulf, state, and time periods. In addition, the jobs/activities/tasks were associated with different degrees of weathered oil. For example, workers patrolling the shoreline were exposed to oil that was more weathered than the oil experienced by workers on vessels offshore. Composition was used in the priors in the Bayesian analyses to estimate exposures for the various exposure groups, but the actual priors were established empirically based on overarching correlations between THC and the BTEX-H concentrations observed in the measurement database. Composition was also used in the estimation of dermal exposure levels.
	Vapor pressure (VP)	The VPs of the pure BTEX-H chemicals at 25°Care 94.8, 24.8, 9.6, 7.72, 153 mm of Hg, respectively. With crude oil weathering of 0%, 20% and 40%, the benzene VPs are 0.864, 0.530 and 0.121 mm of Hg, respectively. The corresponding toluene VPs are 0.620, 0.595 and 0.0.470 mm of Hg. The corresponding ethylbenzene VPs are 0.027, 0.028 and 0.028 mm of Hg, and xylene’s VPs are 0.157, 0.168 and 0.173 mm of Hg. N-hexane VPs are 7.690, 3.469 and 0.267 mm of Hg. The corresponding THC VPs are 29.900, 16.482 and 6.162 mm of Hg.Vapor pressure was not used in the development of EGS but was used in the estimation of dermal exposure levels.
	Temperature	The temperature refers to the temperature of the oil and not ambient temperature. Every approximately 10°C (18°F) increase in the liquid crude oil temperature results in an approximate doubling of vapor pressure. As a point of reference the average water temperature at Grand Island, LA in April is about 70°F (21°C) and in July the water temperature is typically about 85°F (29°C). This means with everything else constant, the vapor generated from the fresh crude oil in April was about ½ the concentration that was generated from fresh oil in mid-July when the well was mechanically capped. Temperature was reflected in the determinant of time period.
	Surface area	The emission rate from a source is dependent on several factors including the surface area of the pool of the agent. The configuration of the pool (length versus width) is important but in general, the greater surface area, the greater the emission rate.Surface area applies to both oil in a pool on the water surface or in a plume under the water surface. It also covers the surface area of a transfer line during a connect/disconnect. The surface area of the oil on the water varied. In addition, the connect/disconnecting of dispersant-containing equipment and dispersant spills could have differently sized pools. The size of the oil pool was used in the modelling of dispersant vapor exposure levels.
Workplace: Job/activity/task	Emission points	The characteristics of the emission points (i.e. number of emission points, overall size of the emissions, etc.) varied across jobs on the rigs and across the activities and tasks of the workers on the vessels and on land. For some jobs/activities the exposure was attributed to both being in an area where the ambient air was contaminated and working with the oil related agent of interest (e.g., benzene) or the agent itself (such as 2-butoxyethanol), while for other jobs/activities exposure was attributed to being in the area where the agent(s) was present, but no direct contact occurred.
		For example, workers in many rig jobs had direct contact with oil-contaminated equipment, while other rig workers received their only exposure from the ambient air outside on the rig deck. Job title was used as a determinant value for rig vessel EGs, but not on any other EG because such information was not available in the monitoring data. Information was available on what activities different vessels performed and we used vessel activity as a determinant (see Workplace: vessel/vessel type below). On some vessels, some jobs personally and directly performed an oil-related activity (such as handling oily boom) and thus were closer to the emission source, whereas other jobs on the same vessel did not perform any oil-related activity and thus were further from the emission source. These personally performed activities resulted in additional determinant values. Various tasks provided other determinant values. They were generally thought to be less than full-shift work, such as the connect/disconnect of transfer lines containing oil. The differentiation of job/activity/task was only for evaluation purposes and did not affect exposure levels per se. Finally, mechanisms of release were distinguished among decontaminating activities (i.e. pressure spraying, or cleaning with cloth and other absorbent materials) for THC, BTEX-H, and oil mist.
	Ventilation	Most activities and tasks were performed outdoors, with the primary type of ventilation being natural dilution. Some of the rig vessels had inside areas not associated with living quarters such as engine rooms, storage areas, mud rooms, etc. where air changes were controlled by heating/ventilation/air conditioning equipment. Entry into a tank, by regulation, is considered a confined space, with mechanical ventilation required. The presence or lack of ventilation was incorporated by the job/activity/task determinant and not used as a distinct determinant. Ventilation was not considered for developing EGs but air changes per hour in likely work areas was considered for estimating exposure levels to dispersant vapor concentrations.
	Wind	The wind direction and speed varied over time. Wind was not considered for EGs but was incorporated in the estimation of exposure levels to dispersant vapors and PM2.5 concentrations.
	Pressure differences in an area	The inside areas of the rig vessels, i.e. the living quarters and offices were under positive pressure. Jobs located inside the rigs were grouped and distinguished from jobs located outside.
Work Practices of activities/tasks	Location of the worker (relative to the emission points, ventilation and wind)	Activities performed by workers on vessels and on land were at different distances from the wellhead. Distance across the Gulf waters was particularly important for the PM2.5EGs. Distance was not used in the development of EGs for dispersant vapors but was used in the estimation of dispersant vapor concentrations.
	Duration and frequency of an activity/task	Duration and frequency affect total exposure (i.e. cumulative exposure). The number of in situ burns per day, the number of days burns occurred and the number of days that oil and gas were flared were considered in the PM2.5exposure estimation process. Additionally, the number of days dispersant was applied aerially or by vessel was considered in developing the dispersant exposure estimates. The number of days was considered in the estimation of cumulative exposure. These metrics were used as a determinant in the calculation of cumulative exposure, but not in the development of EGs.
	Other Agents	Collected oil and oily water, cleaning or other support chemicals, and contaminated equipment contain varying concentrations of the agents being studied, but at levels different from those levels observed in the weathered crude oil. In some cases, especially in the cases of fuels and dispersants, these agents were transferred from land to vessels and planes and from vessels to other vessels. Workers connected and disconnected transfer lines with different chemicals, as they also pumped and performed maintenance on pumping equipment containing different chemicals. We distinguished between working with oil- and dispersant-containing equipment, but did not consider other agents for other EGs.
Workplace: Vessel name, vessel type and (vessel) activity	Emission points	The 2 rigs that stopped the oil release were performing different activities with different equipment from all other vessels. Two other rigs were drilling new wells. Other vessels in the same area were piloting the remotely operated vehicles (ROVs). Several large and smaller vessels were located throughout the Gulf waters had different functions or performed different activities (e.g., transport, scouting, skimming, burning, and boom handling). The rig, the ROV, fire burner control and research vessels were identified by name. Workers who identified such vessel-related activity in open-ended questions but provided no vessel name, was assigned the determinant value of “All ROVs” or “All RVs” (this situation did not occur for the rig or fire burner control vessels). Other vessels performing a single activity were expected to have had different emission characteristics, but due to the large number of vessels and the lack of information on the vessel names and their characteristics from the study participants, we identified these by vessel type (barge, draft/air/jon boat) or activity (transport, scouting, etc.). Activity was a determinant for EGs where the workers on the vessel were expected to have the same distribution of exposures (vs. vessels where some participants had closer contact with the agent (Workplace: job/activity/task above)).
	Composition	The rig vessels and ROV vessels stayed in the immediate area of the well where the vapor concentrations of the oil constituents were the highest and thus was the least weathered. Also, these vessels had the highest PM2.5 levels associated with the flaring of oil and gas by two of the rig vessels. Vessels, such as skimmers, were in direct contact with the oil but it had been weathered, resulting in lower oil constituent concentrations than those experienced on the rig vessels. Weathering would be incorporated into time (see Dates (time periods) below). Weathering was used in the estimation of dermal exposure.
	Distance from the source	The rig vessels were very large, with the main deck being about 30 m above the water surface, while the ROV vessels had decks about 10 m above the water surface and other vessels such as fishing and scouting vessels had decks about 3 m above the water surface. Air/jon/draft boats were <1 m above the water surface. In addition, the rig and ROV vessels worked within 5 nmi of the wellhead; whereas fishing and scouting vessels worked throughout the Gulf and air/jon/draft boats worked near shore. Distance from the source likely was incorporated into vessel name, type of activity and so was not used as a separate determinant.
Area/State	Distance from the source	Generally, the oil had undergone greater weathering the further the distance from the leaking well. Some oil remained in underwater plumes that surfaced beyond the immediate area of the wellhead. In these cases, although some weathering occurred due to dissolution, significant weathering may not have occurred until the oil surfaced. Similarly, PM2.5 levels decreased inversely to the distance from vessels burning oil in situ and the rigs flaring oil/gas. We divided the Gulf of Mexico into 3 areas: a combined hot zone and source (“could see the wellhead”), offshore and near shore (see text for definition). In addition, the 4 Gulf states in the study were distinguished for their distance from the wellhead: LA, MS, AL, and FL. We identified a forth area (hot zone alone) for PM2.5.
	Duration and frequency	The duration and frequency of activities and tasks likely varied by Gulf state (and likely at even finer resolution than state). Because we had no information on such differences, we did not adjust for them for water workers. These determinants are reflected in the use of state as a determinant for land workers.
	Emission points	For the most part, emission points were contained within the concept of activities, locations and/or time periods and so were not considered separately. The characteristics of emission points on the beaches (primarily pools of weathered oil and tar balls) were different from those at ports and docks (primarily oil-contaminated boom and other equipment). Activities occurring at the beaches and at the ports and docks were identified as different determinant values.
Dates (Time Periods)	Degree of weathering	The degree of weathering was the lowest before 15 July 2010 due to the continuous flow of fresh oil. Dispersant was applied by plane and vessel before this date, while after 15 May 2010, dispersant was also injected below the water surface, resulting in more weathered oil reaching the surface. Once the flow of oil was stopped (15 July 2010), the degree of weathering increased with increasing time. Time periods (TPs) were developed reflecting the degree of weathering in the oil and changes in events.
	Vessel name, vessel type, activity	The vessels deployed and the activities they performed changed over time. Almost all vessels were decommissioned by 31 December 2010, while land operations continued (albeit it at a reduced rate). The time periods developed reflected different functions and these broad decommissioning dates.
	Job/activity/task	The jobs/activities/tasks performed changed by time period. Marsh clean-up did not start until after August, 2010. Boom operations were generally over by 31 December 2010. The time periods developed reflected these various changes.