Benzene releases from gasoline stations - Implications for human health: Risks to human health

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• Long-term benezene exposures
• Short-term benzene exposures

Long-term benzene exposures

Evaporative emissions from gasoline stations

In this section, benzene emissions from gasoline stations are modelled based on estimates of total evaporative loss. Air dispersion modelling is used to estimate how benzene concentration changes with distance from the gasoline station fenceline. Estimates of cancer potency are then used to estimate the risk of this benzene exposure to residents living in the vicinity of gasoline stations.

Evaporative emission scenarios were developed to estimate benzene exposure in the vicinity of gasoline stations. A baseline throughput of 1.0 million L/year was selected along with the median and high throughput values of 4.0 and 10.6 million L/year described previously in Gasoline station and delivery tanker truck statistics in Canada.

A total gasoline evaporative loss rate of 0.15% of the throughput of the gasoline station is assumed in this report (Statistics Canada 2012). The scenarios consider 0.6% v/v benzene in liquid gasoline in Canada (ECCC 2021) and a gasoline liquid density of 0.755 g/cm³. The resulting benzene emission rates for the baseline, median and high-throughput gasoline stations are estimated to be 0.25, 1.0 and 2.65 mg/s, respectively. Input factors for the air dispersion modelling calculations are given in Table B-1 of Appendix B.

The benzene emission rates for the baseline, median, and high throughput gasoline stations were used in SCREEN3 (1996) to estimate the dispersion of benzene in air at various distances from the gasoline stations. SCREEN3 is a screening-level Gaussian air dispersion model, which uses wind as the driver for air dispersion of vapours. A maximum exposure concentration was estimated based on a combination of meteorological conditions, including wind speed, air turbulence and humidity. In this work, the model was used to estimate benzene concentrations released into the air from an area source at the gasoline station, and provides the maximum concentrations of the released vapour at a chosen receptor height and at various distances from the source, in the direction downwind of the prevalent wind 1 hour after a given release event. For exposures from area sources over the span of a year, it can be expected that changing wind directions may vary the location that receives the maximum vapour concentrations from the source. The annual concentration received at a fixed location calculated from an area source was determined by multiplying the maximum 1-hr concentration by a factor of 0.2 (SCREEN3 1999; ECCC, HC 2016, 2017). SCREEN3 has been used by Health Canada in air dispersion modelling in a number of screening assessments of petroleum group substances, including those for the Petroleum and Refinery Gases (ECCC, HC 2017) and Natural Gas Condensates (ECCC, HC 2016), which indicated that the concentrations of dispersed vapours at different distances from the release source predicted by SCREEN3 are comparable to measured values and other screening-level air dispersion models.

An exposure scenario was developed for the general population in Canada living in the vicinity of the fenceline of a 20 m × 20 m gasoline station (Figure 2). The combination of the gasoline station refuelling bay and vent stacks was considered an area of emission source and a receptor height corresponding to the average height of Canadians (1.74 m) was used in the air dispersion calculations. The gasoline station and vent stacks are not single point sources of vapour emission. Additionally, the gasoline station cannot be considered a uniform volume source for vapour release as sources of benzene emissions are localized at multiple locations near the ground or at the top of the vent stacks.

The parameters used in air dispersion modelling of gasoline station benzene releases are provided in Table B-1 of Appendix B. The incremental benzene concentration attributable to gasoline station emissions for the baseline, median and high throughput scenarios is presented for varying distances from the fenceline (1.74 m receptor height) in Figure 4 (see also Table B-2 [Appendix B]).

Figure 4 - Text description

Figure 4 is a graph showing 3 plots representing the average annualized benzene concentration (y-axis in µg/m³) attributable to evaporative losses from:

• 1 million L/year throughput gasoline station
• median throughput gasoline station
• 95th percentile throughput gasoline stations at different distances from the fence line (x-axis in metres)

The average urban background benzene concentration of 2.0 µg/m³ is also shown for all distances from the fence line. The y-axis shows average annualized benzene concentrations from 0 to 10.0 µg/m³. The x-axis shows distances from the fence line from 0 to 500 m. The 1 million L/year throughput gasoline station starts at a benzene concentration of ~0.5 µg/m³ at 10 m from the fence line, peaks at 0.88 at 20 m from the fence line and then drops to ~0 at 200 m from the fence line. The median throughput gasoline station starts at a benzene concentration of ~3 µg/m³ at 10 m from the fence line, peaks at 3.5 at 20 m from the fence line and then drops to ~0 at 300 m from the fence line. The 95th percentile throughput gasoline station starts at a benzene concentration of ~8 µg/m³ at 10 m from the fence line, peaks at 9.2 at 20 m from the fence line and then drops to ~0 at 450 m from the fence line. Arrows indicate distances where the benzene concentration from each type of gasoline station is equivalent to the average urban background concentration.

The highest average annualized increases in modelled benzene concentrations occur at a distance of 20 m from the station fenceline, and were estimated to be 0.88, 3.5 and 9.2 µg/m³, respectively, for the baseline, median throughput and high throughput gasoline station scenarios. As residential areas have been identified at distances starting at 10 m from the fenceline of gasoline stations with these throughputs in Canadian locations, there is the potential for residential areas to be situated in areas with the highest estimated benzene levels.

To characterize the potential incremental maximum risks to health from these three scenarios, the highest predicted annual concentrations of benzene were compared with the TC₀₅ of benzene (Canada 1993). One approach that can inform the health risks of ambient concentrations is to calculate the margin of exposure (MOE). In this assessment, the MOE is the ratio of the TC₀₅ effect level benzene concentration (identified in Health effects of benzene) to the observed benzene concentrations. As a reference for the risk associated with the urban background benzene concentration of 0.44 µg/m³, comparing it with the TC₀₅ results in a MOE of 14,700/0.44 = 33,400, which approximates a risk level of 1.5 per million population [1,000,000/(33,400/0.05)] exposed to this concentration of benzene.

Table 1 provides the MOE and relative risk per million for each exposure scenario.

The MOEs of 1,600, 4,200 and 16,700 correspond to increases in incremental risk of cancer of 32, 12 and 3 per million for the high-throughput, median-throughput and baseline-throughput gasoline stations, respectively. These results indicate that estimated exposures to benzene from gasoline stations with annual throughputs greater than 1 million L may pose an elevated risk to human health to those living in close proximity to these gasoline stations compared with exposure within the general population in urban environments. Of particular concern are those living near the median- and high-throughput gas stations, as the maximum incremental increase in risk exceeds 10 per million. The distances from the gasoline station fenceline at which the incremental benzene concentrations from gasoline stations correspond to the 1 in 1,000,000 risk level of 0.29 µg/m³ are estimated as 70 m for the baseline-, 160 m for the median- and 290 m for the high-throughput gasoline station scenarios (Table B-2 and Figure 4).

Short-term benzene exposures

Vapour release from underground storage tanks during tanker truck unloading

In this section, the short-term (1-hr) release of benzene from a gasoline station during fuel unloading by tanker truck is determined. Using air-dispersion modelling, the variation of concentration of benzene at distances away from the gasoline station is calculated. These 1-hr air concentrations are compared with short-term health endpoints. The relative contribution of these short-term releases is qualitatively compared with the total evaporative loss from the station.

It was assumed that the unloading of gasoline from 35,000 L to 43,900 L tanker trucks into the gas station storage tank requires a period of 1 hr and the release of the vapour from the unloading event was averaged over this same period. One hour was also the shortest averaging time of emissions in the dispersion model. As liquid gasoline enters the storage tank from the truck, an equivalent amount of the air and gasoline vapour mixture in the storage tank headspace is released into the atmosphere from the storage tank vent stack to maintain the storage tank pressure. A vent release height of 3.66 m (12 ft) (CAPCOA 1997) for the gas station storage tank was assumed (Figure 1 a). In addition, a 0.3 m × 3 m area corresponding to the approximate release area of the vent stack was assumed, as depicted in Figure 2.

Due to the physical–chemical properties of gasoline components, its liquid composition differs significantly from that of the headspace vapour, which equilibrates on top of the liquid phase in a closed vessel. The California Air Pollution Control Officers Association (CAPCOA) estimates the vapour space above liquid gasoline in underground gasoline station storage tanks to be a 70:30 (volume %) air–gasoline mixture (CAPCOA 1997) with a density of 1.682 kg/m³. For gasoline with a 0.6% v/v composition of benzene in the liquid phase, the air–gasoline vapour mixture released into the atmosphere will contain 0.18% w/w benzene (CAPCOA 1997; Hilpert et al. 2019).

Based on these values, a benzene release rate of 32.7 mg/(m²·s) from the area of the vent stacks is estimated for the average 1 hr of vapour release time associated with the unloading of a 35,000 L capacity truck.

Based on the benzene release rate and the exposure factors given in Table B-3 of Appendix B, the maximum incremental benzene concentrations for 1 hr of exposure at different distances from the vent pipe array without vapour recovery from the unloading of a 35,000 L delivery tanker truck, as calculated by SCREEN3, are shown in Figure 5 and provided in Table B-4 of Appendix B.

The maximum 1 -hr increase in modelled benzene concentrations occur at a distance of 10 m from the station fenceline, and were predicted for unloading of 35,000 L and 43,900 L tanker trucks to be 805 µg/m³ and 1,010 µg/m³, respectively (Figure 5). Residential areas have been identified at this distance from gasoline stations, indicating the potential for residences to be situated in areas with the highest benzene levels.

Figure 5 - Text description

Figure 5 gives the predicted maximum 1-hr incremental benzene concentration as a function of distance from vent stacks as a result of 35,000 and 43,900 L gasoline tanker truck deliveries. There are 2 graphs for the 1-hr incremental benzene concentration (y-axis in µg/m³) attributable to evaporative losses from 1-hr unloading of a 35,000 L truck and 1-hr unloading of a 43,900 L truck, at different distances from the fence line (x-axis in metres). The y-axis shows 1-hr incremental benzene concentrations from 0 to 1000 µg/m³. The x-axis shows distances from the fence line from 0 to 500 m. The California AREL of 27.0 µg/m³ is also shown for all distances from the fence line.

The 35,000 L truck starts at a benzene concentration of 800 µg/m³ at 10 m from the fence line and drops to ~0 at 300 m from the fence line. The 43,900 L truck starts at a benzene concentration of 1000 µg/m³ at 10 m from the fence line and drops to ~0 at 400 m from the fence line. The benzene concentrations from the trucks reach the California AREL levels at ~250 m from the fence line. Arrows indicate distances from the gasoline station where the benzene concentration from the unloading of trucks is equivalent to the California AREL value.

Comparing this estimate of 805 µg/m³ with the effect level of 16 mg/m³ based on developmental hematotoxicity in mice (Keller and Snyder 1986) results in a MOE of 20 (Table 2), indicating a possible short-term risk to human health. For a large delivery truck, the MOE is even smaller at 16. Typically, an acceptable MOE to an effect level determined from laboratory animals is in the range of 300 to 1,000.

The California EPA considered a safety factor of 600 with the above effect level to determine their 1-hr benzene AREL of 27 μg/m³ (OEHHA 2014). The modelled incremental benzene concentrations from releases from the vent stacks become less than this AREL at distances between 210 and 240 m (for the 35,000 L and 43,900 L tanker trucks, respectively) from the gasoline station fenceline.

These results indicate that estimated exposures to benzene during truck unloading may pose an elevated risk to pregnant people and their developing fetuses who live in close proximity to gasoline stations or those within the vicinity of a tanker truck unloading event. The frequency of these elevated short-term exposure events occurring is dependent on station throughput and volume of fuel per delivery. For example, assuming all deliveries are 35,000 L, it is estimated that there would be 29, 114 and 303 such events per year for the baseline-, median- and high-throughput gas stations, respectively.

The transient benzene concentrations released during the 1-hr period of gasoline tanker truck unloading are considerably higher than the 24-hr average concentrations of benzene from continuous releases from the gasoline station. At a distance of 20 m from the fenceline (Table B-4, Appendix B), averaging the concentration of benzene released from the 35,000 L tanker truck unloading event over 24 hrs, with 2 deliveries per week, and accounting for changes in wind direction during tanker truck unloading events on different days [650 µg/m³×(1/24)×(2/7)×0.2], the benzene concentrations from tanker truck unloading are estimated to contribute up to approximately 40% of the total benzene concentration that results from all gasoline station evaporative losses.