5 Ozone
Ozone (O3) is a colourless, pungent, highly reactive gas that is formed as a result of chemical reactions between primary pollutants. It is made up of three oxygen atoms. In the lower atmosphere, ozone is formed through photochemical reactions involving the action of ultraviolet light on the precursor pollutants oxides of nitrogen (NOx) and volatile organic compounds (VOC). Motor vehicles are the main source of NOx in New Zealand and contribute to VOC emissions. Other sources of VOCs include domestic home heating and industrial processes.
5.1 Guideline values
The ambient air quality guideline values (Ministry for the Environment, 2002) for O3 are:
- 150 µgm-3 (one-hour average)
- 100 µgm-3 (eight-hour average).
The 2002 ambient air quality guideline values for O3 are the same as those adopted in 1994 (Ministry for the Environment, 1994).
5.2 Concentrations
Concentrations of ozone have been measured in Auckland and on the outskirts of Christchurch. The location of sampling for the latter study, downwind of the main urban area, was to allow for the chemical reactions of the precursor pollutants to take place prior to measurement.
In Auckland concentrations of ozone have been measured at Pukekohe, Musick Point, Whangaparoa, Mangere and the Skytower. While the eight-hour guideline value for O3 has only been exceeded at one site Musick Point (in 2002), all sites except Mangere have recorded concentrations within the 'alert' air quality category. That is, greater than 66% of the guideline value.
Concentrations of ozone on the outskirts of Christchurch were also within the guideline values with maximum one-hour and eight-hour averages of 97 µgm-3 and 76 µgm-3 at Lincoln, and 93 µgm-3 and 75 µgm-3 at Kainga.
Although monitoring of ozone has not been carried out in other urban centres of New Zealand, a report by McKendry (1996) indicates that Auckland, Christchurch and Hamilton have the greatest potential for elevated ozone concentrations in New Zealand.
5.3 Health effects of ozone
The health impacts of exposure to ozone concentrations have been widely studied using both epidemiological methods and laboratory studies. Dennison et al (2002) summarises the health effects associated with exposure to ozone as:
- increase in daily mortality, respiratory and cardiovascular disease
- increase in hospital admissions and emergency room visits, respiratory and cardiovascular disease
- decrease in lung function
- increase in symptoms of respiratory illness such as cough, phlegm and wheeze
- increase in bronchodilator usage.
At low concentrations ozone can cause tissue injuries in the lungs and can result in significant impairment of pulmonary function. The impact of ozone on health depends on a number of factors including magnitude of concentration, duration of exposure, climate, individual sensitivity and pre-existing conditions. Those most susceptible to concentrations of ozone include children, people with pre-existing diseases, the elderly and healthy adults exercising in the outdoors.
Although some studies have indicated the potential for a no effects threshold, the overall interpretation of the health effects literature is that there is no threshold of exposure, below which effects do not occur (Dennison et al, 2002). A dose response relationship of 0.6% per 10 µgm-3 (eight-hour mean) for mortality and 0.7% per 10 µgm-3 (eight-hour mean) for hospital admissions was used to estimate the impact of ozone concentrations in the Quantification of the Effects of Air Pollution on Health in the United Kingdom study (Department of Health, 1998). This dose response relationship was based on studies carried out in the urban and rural areas of the United Kingdom during the summer months, prior to 1998.
5.3.1 Implications for New Zealand
The health implications of ozone in New Zealand are difficult to determine, as monitoring data are limited to the outskirts of Christchurch and a number of sites within Auckland. An estimate of the order of magnitude impact of ozone concentrations in Auckland on mortality based on the relationship used in the UK Department of Health report (1998) is shown in Table 5.1. Although monitoring for ozone has been carried out at a number of monitoring sites in Auckland the population exposure is uncertain. Estimates of health have been made based on three scenarios to provide a range of health estimates. The population exposure scenarios from which estimates were made were:
1) population exposure approximately equal to the average concentration across all sites
2) population exposure based on the annual average concentration at the worst case site
3) population exposure based on the annual average concentration at the best case site.
To calculate the estimated number of deaths per year in Auckland that may be associated with exposure to ozone, the annual average ozone concentration for each scenario was divided by ten to be consistent with the units for the dose response relationship. This value was multiplied by the annual mortality rate for Auckland, which was estimated at 7118 based on the average for 1998 and 1999. This value was multiplied by the dose-response relationship described by UK Department of Health (1998) to give an estimate of the annual mortality. Equation 5.1 shows the calculation for the estimate based on average exposure of 39 µgm-3.
Equation 5.1: 3.9 x 7118 x 0.007 = 195
Estimates are based on the assumptions that the dose-response relationships established for the United Kingdom in the COMEAP (1998) study are applicable to ozone exposure in Auckland and that the measured ozone concentrations in Auckland provide a reasonable indication of exposure in the city. The validity of these assumptions are uncertain and results should therefore be treated with caution.
These calculations indicate that the potential mortality (deaths
brought forward) for Auckland may be over 100 per year.
Table 5-1: Mortality estimates for different estimates of ozone exposure in Auckland
| O3 (average) µg m-3 | Total mortality / year | O3 related mortality/ year | |
|---|---|---|---|
|
Average (all sites) |
39 |
195 |
|
|
Maximum (Skytower) |
48 |
239 |
|
|
Minimum (Mangere) |
27 |
135 |
|
|
Total mortality (average 1998 and 1999) |
7118 |
