7 Benzo(a)pyrene
Benzo(a)pyrene (BaP) is a five ring polycyclic aromatic hydrocarbon (PAH) and is found in combustion generated fine particulate matter. The International Agency for Research on Cancer (IARC) considers BaP a known animal carcinogen and probable human carcinogen (Group 2A).
The main source of BaP in New Zealand is domestic home heating, in particular wood burning. Other potential sources include mobile sources, outdoor burning and rubber tyre wear. Indoor sources of BaP include smoking and open fires and wood burners.
In addition to inhalation of benzene, exposure can occur through dermal absorption and ingestion of food and water.
7.1 Guideline values
The ambient air quality guideline value (Ministry for the Environment, 2002) for benzo(a)pyrene is 0.0003 µgm-3 (annual average). 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 BaP.
7.2 Concentrations
Monitoring of BaP concentrations in New Zealand is limited to a small amount of sampling carried out in Auckland during 1997-1998 and a study period in Christchurch 1999. An estimate of annual average BaP concentrations from the former study was not possible, owing to the poor detection limits of the analysis method. Results from the Christchurch study suggest annual average concentrations of BaP of at least 0.004 µgm-3, over 10 times higher than the ambient air quality guideline value. A strong correlation between BaP and PM10 was found (Gunatilaka, 2002). It is likely that concentrations of BaP are also of concern in other urban areas of New Zealand where PM10 concentrations exceed ambient air quality guideline values as a result of solid fuel burning for domestic heating.
7.3 Health effects of benzo(a)pyrene
While laboratory studies show that BaP is a known carcinogen in animals, epidemiological studies have only been able to assess the effect of a mixture of PAHs, including BaP found in soots, tars and oils. Mutagenicity refers to the ability of a chemical to induce mutations in DNA and in living cells. Benzo(a)pyrene is a promutagen, which means it needs to be metabolised before it can induce mutation. Benzo(a)pyrene can also react with ozone to produce strong mutagens such as benzo(a)pyrene-4,5 oxide (Californian Air Resources Board, 1994).
The IARC classification for BaP is Group 2A, probable human carcinogen. The US EPA has classified BaP as a Group B2 carcinogen of medium potency.
Like benzene, BaP has an inferred effects threshold of zero and the evaluation of 'acceptable risk' is used in determining appropriate ambient air guideline value concentrations. The World Health Organisation (1996) has determined an inhalation unit risk of 8.7 x 10-2 per 1 µgm-3 BaP. This was based on interpolation from risk estimates for PAHs in coke oven emissions (Chiodo et al, 2002). In addition, WHO has also determined an inhalation unit risk from studies of animals exposed to complex mixtures of PAHs. However, in assessing ambient air quality guideline values for New Zealand, Chiodo et al (2002) conclude that the human based risk of 8.7 x 10-2 per 1 µgm-3 BaP is appropriate.
7.3.1 Implications for New Zealand
It is difficult to assess the health implications of existing BaP concentrations in New Zealand because existing ambient air monitoring is limited to Auckland and Christchurch. It was not possible to establish an estimate of annual average BaP concentrations from the Auckland data because of the limitations of the analytical method. In Christchurch, annual average BaP concentrations of around 0.004 have been estimated based on monitoring in the St Albans area.
Table 7.1 shows an estimate of impact of BaP concentrations on the Christchurch population for two separate assumptions. The first (exposure 1) assumes that the concentrations measured in St Albans represent average exposure for the flat urban areas of the city. The second (exposure 2) is based on the assumption that average exposure across the city is 70% of the concentrations measured at St Albans. Both estimates are based on the assumption that all of the existing population of Christchurch City, around 280,000 will be exposed to elevated ambient air concentrations of contaminants.
Table 7-1: Estimates of the potential impact of BaP concentrations in Christchurch
| Estimated annual average BaP µgm-3 | Population exposed | Estimated no. of people affected at risk of 8.7 x 10-2 | |
|---|---|---|---|
|
Christchurch (exposure 1) - as measured at St Albans |
0.004 |
280,000 |
97 |
|
Christchurch (exposure 2) - 70% of the St Albans concentrations |
0.0028 |
280,000 |
68 |
|
If the guideline values were met |
0.0003 |
280,000 |
7 |
