6 Benzene
Benzene is an aromatic hydrocarbon that is produced by combustion and as a result of evaporative emissions. Benzene is used in the manufacture of plastics, detergents, pesticides, and other chemicals. In its most common form, benzene is a liquid that is clear, sweet smelling and highly combustible. Benzene evaporates quickly in the air and is partially soluble in water.
Sources of benzene in ambient air include motor vehicle emissions, domestic home heating, outdoor burning and industry. The main sources of benzene in ambient air in New Zealand are motor vehicle emissions, both tailpipe and petrol evaporation, and domestic home heating emissions. Other combustion processes may also contribute to ambient air benzene concentrations. In indoor environments, smoking can be a major contributor to benzene concentrations.
6.1 Guideline values
The ambient air quality guideline values (Ministry for the Environment, 2002) for benzene are:
- 10 µgm-3 (annual average) existing
- 3.6 µgm-3 (annual average) from 2010.
Previously, the 1994 ambient air quality guideline values for New Zealand (Ministry for the Environment, 1994) did not include an ambient air quality guideline value for benzene.
6.2 Concentrations
Concentrations of benzene in New Zealand have been measured in Auckland, Christchurch, Hamilton, Dunedin and Nelson. The monitoring has been carried out at the ambient 'residential neighbourhood' sites in all areas as well as 'traffic peak' sites in both Christchurch and Auckland. At the ambient air quality monitoring sites, annual average benzene concentrations have ranged from around 1 µgm-3 to around 5 µgm-3. Annual average benzene concentrations in excess of 16 µgm-3 have been measured at 'traffic peak' monitoring sites at Khyber Pass Road (Auckland) and Riccarton Road (Christchurch).
6.3 Health effects of benzene
Benzene is a known carcinogen and has been classified as a Group A carcinogen of medium potency by the US EPA, and a Group 1 carcinogen by the International Agency for Research on Cancer (IARC). Benzene also has haematological effects and is mutagenic. The major effect of long-term exposure to benzene is on the blood. Benzene causes harmful effects on the bone marrow and can cause a decrease in red blood cells leading to anaemia. With exposures from less than five years to more than 30 years, individuals have developed, and died from, leukaemia. It can also cause excessive bleeding and can affect the immune system, increasing the chance for infection. Short-term exposure to high levels of benzene can cause drowsiness, dizziness, unconsciousness and death.
An overview of international studies on animal and human exposures to benzene is summarised in the Ministry for the Environment's Health effects of Eleven Hazardous Air Contaminants and Recommended Evaluation Criteria (Chiodo and Rolfe, 2002).
Because benzene is a carcinogen, it has an inferred effects threshold of zero. Guideline values for contaminants for which there is no safe level are normally based on the evaluation of 'acceptable risk'. A number of different risk models have been used to derive the risk of developing leukaemia from benzene exposure. The World Health Organisation (1996) indicates a range of 4.4 x 10-6 to 7.5 x 10-6 for estimates of excess lifetime risk of leukaemia at an ambient air concentration of 1µg/m3 (unit risk). Other risk estimates for benzene inhalation include the US EPA unit risk estimate of 8.3 x 10-6, and the California Air Resources Board value of 29 x 10-6 (Chiodo et al, 2002).
6.3.1 Implications for New Zealand
Although ambient air quality guideline values for benzene are complied with in most areas of New Zealand, even concentrations of benzene below ambient air quality guideline values will have some impact. The unit risk estimates provided by World Health Organization (1996), the United States Environmental Protection Agency and the California Air Resources Board can be used in conjunction with air quality monitoring data for benzene to estimate the potential impacts of existing benzene concentrations for the main urban areas of New Zealand.
Table 6.1 shows an estimate of annual average benzene concentrations in Auckland, Hamilton and Dunedin. Auckland data are based on average concentrations measured in the Penrose, Henderson and Mt Eden for 2002 and do not include data for roadside monitoring sites, e.g. Khyber Pass. Hamilton and Dunedin data are based on the Ministry of Health 1996/1997 benzene monitoring (Stevenson and Narsey, 1998). These estimates are based on an average across the ambient air quality monitoring sites used in the study and do not include data for roadside monitoring sites. For Christchurch, the estimated annual average is based on more recent monitoring (Gunatilaka, 2002), which suggests lower average concentrations (average of ambient sites = 3.3 µgm-3) than those measured during the 1996/1997 study (average of ambient sites = 4.2 µgm-3). Note that estimates are not based on exposure modelling or detailed spatial modelling and are likely to be subject to a reasonable level of uncertainty.
Table 6.1 also shows the leukaemia risk for the existing population in each area based on the assumption of an average exposure to the estimated benzene concentrations throughout the study area. Some adjustments have been made to population data in some areas to better reflect likely exposure. For example, the Port Hills and outer suburbs of Christchurch were not included in the population exposure estimates.
Table 6-1: Estimates of benzene risk to populations in Auckland, Christchurch, Hamilton and Dunedin
