The Impact of Revising the Benzene Threshold Exposure Limit

The Impact of Revising the Benzene Threshold Exposure Limit

It is estimated that nearly 1.37 million workers are exposed to benzene at work.

The American Conference of Governmental Industrial Hygienists (ACGIH) proposes to reduce the eight-hour time-weighted average exposure threshold limit value (TLV) for benzene to 0.02 ppm and reduce the 15-minute short-term exposure limit (STEL) to 0.1 ppm. So, what is known about workers who were exposed to benzene, and how does this change the paradigm of what is already known? The U.S. National Occupational Exposure Survey (1981–1983) estimated that about 272,300 workers (143,000 women) were potentially exposed to benzene in the U.S. alone. Due to economic growth, technological and innovations in industry over the past 40 years, the current exposed population may be closer to 1.37 million workers nationwide. 

Sampling data collected by the OSHA Integrated Management Information System (IMIS) shows limited evidence of data across the spectrum of industrial markets. The data collected shows that most workers (94.5 percent) would be exposed to benzene above the proposed ACGIH TLV. Only 2.9 percent of workers were exposed above the current ACGIH TLV of 0.5 ppm and 1.6 percent of workers were exposed above the OSHA Permissible Exposure Limit (PEL) of 1 ppm as an eight-hour TWA. So why has only limited evidence been collected to evaluate the workforce? Benzene is considered a confirmed human carcinogen by ACGIH, International Agency for Research on Cancer (IARC), National Toxicology Program (NTP) and the U.S. Environmental Protection Agency (EPA). Global burden of cancer is high and continues to increase based on epidemiological data. The evidence is clear that all workplaces should be reassessed for benzene vapor to determine occupational exposure if the ACGIH TLV and STEL are reduced. 

Introduction to Benzene 

Benzene or benzol is a clear, colorless, highly flammable liquid with a kind of sweet odor. It is an aromatic organic compound with the molecular formula C6H6. The chemical is abundant as a component in gas emissions in motor vehicle exhaust, volcanic or forest fires and it is found in gasoline, crude oil and cigarette smoke. 

Benzene is a component of many industrial products derived from coal and petroleum and it is found in gasoline and other fossil fuels. Trace amounts of benzene can be found in chemicals like toluene, ethylbenzene and xylene, and it is used to make chemicals like cumene and cyclohexane. Benzene is used in the manufacture of plastics, detergents, pesticides, synthetic fibers, rubber, lubricants, dyes, resins, drugs and so many other industrial chemicals. Additionally, ink and variety of painting products, such as spray paints, base and top coat paints, sealers, lacquers and stains, also contain some portions of benzene.  

Benzene is widely used in the U.S. It ranks in the top 20 chemicals for production by volume. Most people can begin to smell benzene in air at approximately 60 parts per million parts of air (ppm) and recognize it as benzene at 100 ppm. According to the National Institute for Occupational Safety and Health (NIOSH), the odor threshold level is 4.68 ppm in air. 

Benzene in the Environment 

Benzene is also commonly found in the environment. Benzene levels in the air can be elevated by emissions from burning coal and oil, benzene waste and storage operations, and evaporation during transfer at gasoline service stations. Tobacco smoke is another source of benzene in the air, particularly indoors. Industrial discharge, disposal of products containing benzene and gasoline leaks from underground storage tanks release benzene into water and soil. Indoor air generally contains levels of benzene higher than those in outdoor air. The benzene in indoor air comes from products that contain benzene such as glues, paints, furniture wax and detergents. 

Historical Perspectives of Benzene 

Industry has known that benzene is dangerous to workers for a long time. In 1928, medical experts recognized a connection between benzene exposure and leukemia. In 1938, a study of benzene-exposed workers in the printing industry showed high levels of blood disease. A 1948 study published by the American Petroleum Institute (API) showed that benzene can cause leukemia. The API study concluded that the only “safe” level of benzene exposure is no exposure at all. By about the end of World War II, the ACGIH recommended limits on industrial exposure to the chemical, recognizing health risks associated with exposure. 

Historically, benzene was used as a degreaser of metals and a solvent for organic materials. Benzene occurs naturally in petroleum products (e.g. crude oil and gasoline), and is also added to unleaded gasoline for its octane-enhancing and anti-knock properties. Typically, the concentration of benzene in these fuels is 1 to 2 percent by volume. Gasoline can be enriched with benzene by adding ethyl benzene-toluene-xylene, which is generated during coke making. Where necessary, side stream petroleum is added to adjust the octane rating; for example, reformate includes 5 to 12 percent benzene.

The primary use of benzene today is in the manufacture of organic chemicals like styrene, phenol, cyclohexane, aniline, maleic anhydride, alkylbenzenes and chlorobenzenes. It is an intermediate in the production of anthraquinone, hydroquinone, benzene hexachloride and benzene sulfonic acid. The second-largest use of benzene in the U.S. (accounting for 22 percent of demand) is in the manufacture of cumene (isopropyl benzene), nearly all of which is consumed in phenol production. Also, benzene is used to make chemical intermediates: cyclohexane, nylon monomers (15 percent); nitrobenzene, intermediate for aniline and other products (7 percent); alkylbenzene, detergents (2 percent); chlorobenzenes, engineered polymers (1 percent); and miscellaneous uses (1 percent).

Occupational Health Effects from Exposure 

Individuals employed in industries that make or use benzene may be exposed to benzene. These industries include benzene production (petrochemicals, petroleum refining, and coke and coal chemical manufacturing), rubber tire manufacturing, storage or transport of benzene and petroleum products containing benzene. Other workers exposed to benzene include coke oven workers in the steel industry, printers, rubber workers, shoe makers, laboratory technicians, firefighters and gas station employees. 

Several factors determine whether harmful health effects will occur, as well as the type and severity of such health effects. These factors include the frequency and length of time of the exposure. Most information on effects of long-term exposure to benzene is from studies of workers employed in industries that make or use benzene. These workers were exposed to levels of benzene in air far greater than the levels normally encountered by the general population. Current levels of benzene in workplace air are much lower than in the past with the advent of specific OSHA compliance requirements. 

The major effect of benzene from long-term exposure is on the hematopoietic system. Long-term exposure means exposure of a year or more. Benzene can cause harmful effects on the bone marrow by decreasing the production of red blood cells, leading to various forms of leukemia. It can cause excessive bleeding and affect the immune system, increasing the chance for infection. 

Some women who breathed high levels of benzene vapor for many months had irregular menstrual periods and a decrease in the size of their ovaries. It is not known whether benzene exposure affects the developing fetus in pregnant women or fertility in men. Animal studies have shown low birth weights, delayed bone formation and bone marrow damage when pregnant animals breathed benzene. 

Benzene can penetrate through intact skin. The amount of benzene absorbed through the skin (e.g., hands) depends on the solvent. Even at current contamination levels, which are less than 0.1 percent in most products, the amount of benzene absorbed over a long period can be significant, depending on exposure time and skin surface area.

With occupational exposures less than five years to more than 30 years, individuals over time died from cancer including leukemia and multiple myeloma. Long-term exposure to this substance causes neurological symptoms and affects the bone marrow causing aplastic anemia, excessive bleeding and damage to the immune system. Benzene is linked to an increased risk of developing lymphatic and hematopoietic cancers, acute myelogenous leukemia, as well as chronic lymphocytic leukemia. 

The early experiences of high occupational exposures led to the identification of hematopoietic effects of benzene and the need for improved control and regulation. As with most occupational standards, a reduction in exposure limits requires air monitoring, engineering and administrative controls along with respirators and personal protective equipment, training and education, medical surveillance and biological monitoring. In 1946, the U.S. occupational exposure limit for benzene, promulgated by the ACGIH, was 325 mg/m3 [100 ppm], but now most European and North American countries have harmonized at 1.63-3.25mg/m3 [0.5-1 ppm]. 

The IARC classifies benzene as “carcinogenic to humans,” based on sufficient evidence that benzene causes acute myeloid leukemia. IARC notes that benzene exposure has been linked with acute lymphocytic leukemia, chronic lymphocytic leukemia, multiple myeloma and non-Hodgkin lymphoma. 

The NTP is a tripart of several different U.S. government agencies, including the National Institutes of Health, Centers for Disease Control and Prevention and Food and Drug Administration. NTP has classified benzene as “known to be a human carcinogen.” The EPA maintains the Integrated Risk Information System, which is another electronic database with information on human health effects and exposure to various substances in the environment. The EPA classifies benzene as a known human carcinogen. 

Global Occupational Exposure Limits 

The original OSHA PEL of 10 ppm was based on the association of benzene exposure with aplastic anemia and the risk of cancers. The eight-hour time-weighted average (TWA) OSHA PEL and 15-minute STEL for general industry are 1 ppm [3.19 mg/m3] and 5 ppm [15.95 mg/m3], respectively (29 CFR 1910.1028). These OELs also apply to the construction and maritime industry. 

Reported deaths at low levels of benzene exposure were associated with multiple myeloma and a long latency period, instead of leukemia, which justified further lowering of the exposure limit. At that time, the risk assessment model was found to be non-significant for response at very low levels of exposure. A re-evaluation of the exposure data indicates that past evaluations may have underestimated the risk and the need to further reduce the OELs. 

The European Union and most European countries have an OEL of 1 ppm, as does the Scientific Committee on Occupational Exposure Limits, but a few countries have opted for lower exposure values. In Germany, the MAK-Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area proposed an acceptable risk corresponding to an eight-hour TWA of 1.9 mg/m3 [0.59 ppm] and 0.02 mg/m3 [0.01 ppm].

ACGIH TLV Chemical Substances Committee recommended that benzene be considered an A1 carcinogen and lowered the TLV during an eight-hour TWA to 0.5 ppm [1.6 mg/m3] and a STEL of 2.5 ppm [~8 mg/m3]. NIOSH recommended exposure level for the 8-hour TWA is 0.1 ppm [0.32 mg/m3] and the STEL is 1 ppm [3.19 mg/m3].  

In 2022, ACGIH published a Notice of Intended Change to modify the TLV to 0.02 ppm for an eight-hour TWA and 0.1 ppm for a 15-minute STEL. Significantly lowering the OELs based on updated epidemiological and toxicological data may trigger the need for further exposure risk assessments where previous information suggested healthy workers were considered protected. 

OSHA Integrated Management Information System 

OSHA compliance safety and health officers (CSHOs) often take personal or area air samples when monitoring worker exposures to chemical hazards. Samples are submitted to the Salt Lake Technical Center for analysis using a vetted analytical standard. IMIS is a resource to compile the OSHA sampling data.  

Between 1984 and 2020, OSHA compliance sampling information on benzene in the IMIS database was reviewed. Workers across a spectrum of industries were evaluated for their benzene exposure by OSHA CSHOs. The frequency and duration of each exposure varied by industry, operation and process.  

Personal sampling results represent workers who wore a sampling device. Area samples are taken in a fixed location and results represented the potential risk from airborne contaminants to workers in that area. Bulk samples help verify if benzene was present in the product. Bulk samples are used individually or in conjunction with personal or area samples to help interpret the level of worker risk. Figure 1 shows the average concentration of 73 bulk samples at 0.29 percent, ranging from 0.04 to 1.84 percent, which is above the 0.1 percent OSHA reporting requirement for chemical carcinogens used in products under the OSHA Hazard Communication Standard (29 CFR 1910.1200). Such information should be published in the Safety Data Sheets. 

It is estimated that nearly 1.37 million workers are exposed to benzene at work. The OSHA IMIS database showed 304 personal and area samples were collected and analyzed. The air sample results are presented in the tables below without consideration for sample collection time. The overall exposure was 0.79 ppm [0.01-10.6 ppm] whereas the overall eight-hour time-TWA exposure was 0.184 ppm for both the personal and area samples collected by OSHA. Based on this limited evidence, nearly 94.5 percent of the personal and area samples were above the proposed ACGIH TLV of 0.02 ppm for an eight-hour TWA. 

Conclusions 

The limited sampling evidence by OSHA suggests that most workers are not exposed above the OSHA PEL. As further epidemiological evidence evolves and the ACGIH exposure guidelines are revised downward to protect against various forms of cancer; there is a critical need to re-evaluate worker risk to benzene exposure. Bulk sample results suggest that the concentration of benzene may be above the 0.1 percent (w/w) OSHA reportable threshold as an ingredient or mixture in chemical products. Petroleum products should be investigated to determine the potential for generating occupational benzene vapor exposures above established guidelines.  

If the ACGIH TLVs are modified as intended, further occupational risk assessments will be needed for all workplaces where benzene vapor may be found in indoor and outdoor working environments, operations and processes. Industry and public support may be needed to lower the current OSHA PEL and STEL and improve worker protection based on the evidence provided in the ACGIH TLV documentation. 

This article originally appeared in the October 1, 2022 issue of Occupational Health & Safety.

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