Vehicle Emissions Reduction

Benzene is an aromatic hydrocarbon which is in both exhaust and evaporative emissions from motor vehicles. Benzene in the exhaust, expressed as a percentage of total organic gases (TOG), varies depending on control technology (e.g., type of catalyst) and the levels of benzene and other aromatics in the fuel. It is generally about 3-5%. The benzene in evaporative emissions depends on control technology and fuel composition and characteristics (e.g., benzene level and the evaporation rate) and is generally about 1%.

The EPA has recently reconfirmed that benzene is a known human carcinogen by all routes of exposure. Respiration is the major source of human exposure.

Long-term respiratory exposure to high levels of ambient benzene concentrations has been shown to cause cancer of the tissues that form white blood cells. Among these are acute nonlymphocytic leukemia , chronic lymphocytic leukemia and possibly multiple myeloma (primary malignant tumors in the bone marrow), although the evidence for the latter has decreased with more recent studies.

Leukemias, lymphomas, and other tumor types have been observed in experimental animals exposed to benzene by inhalation or oral administration. Exposure to benzene and/or its metabolites has also been linked with genetic changes in humans and animals and increased proliferation of mouse bone marrow cells. The occurrence of certain chromosomal changes in individuals with known exposure to benzene may serve as a marker for those at risk for contracting leukemia.

The latest assessment by EPA places the excess risk of developing acute nonlymphocytic leukemia at 2.2 x 10^-6 to 7.7 x 10^-6µg/m3. There is a risk of about two to eight excess acute nonlymphocytic leukemia cases in one million people exposed to 1µg/m3 over a lifetime (70 years). This range of unit risk represents the maximum likelihood estimate (MLE) of risk, not an upper confidence limit (UCL).

A number of adverse noncancer health effects, blood disorders such as preleukemia and aplastic anemia, have also been associated with low-dose, long-term exposure to benzene. People with long-term exposure to benzene may experience harmful effects on the blood-forming tissues, especially the bone marrow. These effects can disrupt normal blood production and cause a decrease in important blood components, such as red blood cells and blood platelets, leading to

• anemia - decrease in number of red blood cells
• leukopenia - decrease in number of white blood cells
• thrombocytopenia - decrease in number of blood platelets, which reduces the ability for blood to clot

Chronic inhalation exposure to benzene in humans and animals results in pancytopenia, a condition characterized by decreased numbers of circulating erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (blood platelets).

Individuals that develop pancytopenia and have continued exposure to benzene may develop aplastic anemia, whereas others exhibit both pancytopenia and bone marrow hyperplasia (excessive cell formation), a condition that may indicate a preleukemic state. The most sensitive noncancer effect observed in humans is the depression of absolute lymphocyte counts in the circulating blood.

The European Union has ser ambient limits for benzene in the second daughter directive on CO and Benzene (2000/69/EC).