Benzene is one of EPA’s “Top 10 Air Toxics,” defined by EPA in its 2017 Report on the Environment as those that contribute more than 90 percent of the estimated incremental cancer risk associated with breathing outdoor air pollution. The key points:
- Outdoor exposure to benzene and concentrations of benzene in blood have decreased dramatically over the past two decades.
- Unregulated indoor exposure to benzene continues to exceed regulated outdoor exposure.
- The incidence of leukemia – the greatest health concern associated with exposure to benzene – has not decreased.
Why is Benzene a Concern?
We all know benzene. It’s the sweetish smell we notice filling our gas tanks. It’s on EPA’s Top 10 list because of concern over its association with leukemia in workers exposed to high concentrations over time. And by “high,” we mean ongoing exposures to concentrations that are hundreds of thousands of times higher than we individually experience at the gas stations for a few minutes each month. Thankfully, these high occupational levels have not been experienced in the US for decades.
Let’s take a closer look at the variables affecting health risk due to benzene exposure:
- What are the trends in emissions of benzene over time?
- What are the trends in resulting air concentrations?
- What are the trends in levels of benzene in our bodies resulting from those concentrations?
- What are the trends in health effects resulting from benzene in our blood?
There is good news to report on nearly all fronts. First, some background information.
What Are the Applicable Regulations?
Benzene content in gasoline is limited by regulation (40 CFR § 80.1230). Most recently, in 2008, EPA created a rule specifically targeting benzene emissions from gas stations, which included an extensive discussion of EPA’s rationale for controlling benzene. (73 Fed. Reg. 1,916, Jan. 10)
How Are We Exposed to Benzene?
It’s ubiquitous. It’s even been detected in deep space. After testing air, water, food, and consumer products, the EPA concluded that more than 99% of personal exposure to benzene was via inhaling air – both indoors and outdoors (L. Wallace, 1996). It’s in the air you are breathing right now.
And that exposure is decreasing. EPA data going back to 1990 show that the emissions of benzene in the US have decreased by about 85% in the following two decades, largely due to controlling the amount of benzene in gasoline. Today, our major sources of outdoor exposure to benzene are about evenly split between cars, non-road emissions (e.g., lawnmowers), wildfires, and prescribed burns. The contribution from residential wood combustion – i.e., wood stoves and fireplaces – is increasing.
Source: USEPA NATA Database
Indoor Exposures to Benzene Exceed Outdoor Exposures
But outdoor emissions do not tell the whole story. In 1979, the EPA’s Total Exposure Assessment Methodology (TEAM) assessed our total exposure to contaminants. The EPA’s major findings were published over several years in four volumes. EPA concluded:
“Personal and indoor exposures to these toxic and carcinogenic chemicals are nearly always greater – often much greater – than outdoor concentrations. We are led to the conclusion that indoor air in the home and at work far outweighs outdoor air as a route of exposure to these chemicals.”
EPA then tested several hundred people’s exposure to benzene in air, at work and at home; in urban areas like Los Angeles and Bayonne, NJ; in rural areas; filling up at gas stations; driving to work; and living near petrochemical refineries in California. What they found surprised them (L. Wallace, 1989):
“Now a large study of human exposure to benzene (EPA’s TEAM Study) has been completed, with the surprising result that the main sources of human exposure are associated with personal activities, not with the so-called “major point sources” …[I.e., a] number of sources sometimes considered important, such as petroleum refining operations, petrochemical manufacturing, oil storage tanks, urban-industrial areas, service stations, certain foods, groundwater contamination, and underground storage leaks, appear to be unimportant on a nationwide basis.…Persons living close to the heavy petrochemical and refining operations at New Jersey and Los Angeles had no greater exposure than those living farther away.”
Thus, employees who pump gasoline for a living receive far less exposure to benzene than from other common activities – particularly if they smoke. Then as today, smoking still accounts for half of US exposure to benzene. Other indoor sources include fireplaces and fumes from gasoline and paint in attached garages seeping into homes.
It is a challenge for a health-focused agency to reduce overall exposure when it cannot regulate the major source of benzene exposure – i.e., indoors.
How Much Benzene Is in the Air?
As you would expect, a significant reduction in emissions has resulted in an equally significant reduction in concentrations measured in air. The USEPA’s 2017 Report on the Environment states that the average air concentration of benzene measured in US outdoor air is 0.083 µg/m3 – an 87% reduction compared to USEPA median measurements of benzene in air, 6.0 µg/m3, just three decades ago.
Concentrations of Benzene in Ambient Air Are Decreasing Over Time
Source: USEPA
By comparison, the short, intense exposure many millions of Americans get each time we fill up our tank – measured as high as 3,000 µg/m3 in 1986 – only happens for an estimated 70 minutes a year. In light of this, it is perhaps not surprising that no correlation has ever been established between pumping your own gas and developing cancer, even among sensitive subgroups.
Why the Steep Decline?
There are several reasons. First, there were 152,995 retail fueling sites in the United States in 2013 — a steep 25% decline from the high of 1994, when the station count topped 202,800 sites. Small corner gas stations are increasingly replaced with mega-stations like Costco, even as vehicle miles increase. Miles driven in the US nearly tripled from 1970 to 2016.
Second, although their size may be increasing, these gas stations are also emitting fewer air toxics over time, initially because of vapor recovery methods like the rubber seals seen on the nozzles in some states. Through redesign, cars are now recovering 75% of gasoline fume emissions, and the rubber seals on gas pump nozzles are interfering with vehicle vapor recovery. In 2012, the EPA told its regions that the rubber seals may be creating a “disbenefit” to air quality and rescinded the requirement for “Stage 2” seals.
Third, the concentration of benzene in gasoline is decreasing over time. It now constitutes less than 1% of gasoline purchased today.
So How Much Benzene Is in Our Bodies?
Less and less over time, measured as benzene in blood. In 2015, the National Cancer Institute (NCI) reported that the 95th percentile for benzene in blood in those over 20 years old was 0.3 nanograms per milliliter of blood as of 2006. In other words, 95% of the US population has less than 0.00000001 ounces of benzene per quart of blood.
The NCI has not established a “Health People 2020” target for benzene, possibly because levels are already so low.
Are Leukemia Rates Decreasing Also?
Surprisingly, no. Leukemia is a form of cancer associated with exposure over time to (among other causes) levels of benzene thousands of times higher than in typical US air. It is also a fairly common form of cancer; approximately 1.5% of men and women in the US – about one person in 75 — will be diagnosed with leukemia at some point during their lifetime. Improved cancer treatment may be reducing the number of leukemia deaths slightly, but the number of new cases has remained essentially the same since at least 1992, according to the National Cancer Institute:
New Cases of Leukemia and Mortality, 1992-2014 (NCI 2017)
Source: National Cancer Institute
Yet the significant reductions in emissions of and exposures to benzene — indoor and out, permitted and unregulated — has not resulted in concomitant reductions in leukemia.
Why Aren’t Leukemia Rates Decreasing Given Significant Reductions in Emissions and Exposure?
One possibility is that the typical amount of benzene we are exposed to is very low to start with, rendering large decreases less meaningful. Looking closely at the graphs, a 90% reduction – from ~10 parts per billion to ~1 ppb – reflects very low concentrations to begin with. To understand the small proportions in air, think of a decrease of 10 drops of water to 1 in an Olympic-sized pool.
In contrast, workers were once exposed to many thousand parts per billion day in and day out for a working lifetime. Today, the Permissible Exposure Limit set by OSHA is 3,200 µg/m3 (1 ppm) for an 8-hour workday and 40-hour workweek – a thousand times higher than the average concentrations we breathe daily.
Do We Need to Control Indoor Exposure to Benzene?
No. The data do not suggest a greater need to regulate indoor exposure to benzene, as the blood levels of benzene have already declined to concentrations well below thresholds of health concern.
The lack of association between exposure to benzene and leukemia over time suggests that routine exposure to benzene is not a significant cause of leukemia relative to other “leukemogens” – substances causing leukemia.
What’s the Big Picture?
Regulated emissions of benzene have reduced dramatically over the past three decades, resulting in significant declines of benzene in outdoor air. Levels of benzene in blood – reflecting both outdoor and indoor exposures – are well below thresholds of health concern. The incidence of leukemia, however, remains flat. Significantly reducing our total environmental exposure to benzene is not reducing the incidence of leukemia.
Quantifying the benefits of further reductions in benzene emissions and exposures may prove challenging, as will maintaining benzene’s reputation as an important air toxic given the success in reducing our exposure.
Dr. Kathryn E Kelly is President of Delta Toxicology, Inc., of Portland, OR. An abbreviated version of this article appears in the American Bar Association’s Section of Environment, Energy and Resources (SEER) June 2018 newsletter.
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