5 Risk Assessments for New Zealand
A limited number of risk assessments have been carried out in New Zealand to estimate the impact of concentrations of PM10 on mortality and morbidity. The largest of these, focuses on the number of deaths per year throughout New Zealand that may be associated with PM10 concentrations from motor vehicle emissions, but also includes estimates of the impacts of PM10 from other sources. The other two studies are risk assessments for Christchurch and Nelson, and are based on the relationships observed in the Hales study (1999), local mortality and morbidity data and measured PM10 concentrations in those areas.
Differences in methodology between the New Zealand motor vehicle impact study (Fisher et al, 2002) and the two risk assessments carried out in Christchurch and Nelson include the following.
- The New Zealand motor vehicle study is based on Kunzli et al (2000) and applies a 4.3% increase in mortality for every 10 µgm-3 increase in annual average PM10 concentrations above 7.5 µgm-3, whereas the Nelson and Christchurch studies are based on a 1% increase in daily mortality for every 10 µgm-3 increase in 24-hour average PM10.
- The Christchurch and Nelson risk assessments include estimates of morbidity effects. While estimates of morbidity effects are included in Kunzli et al (2000), no estimates for New Zealand were made in Fisher et al (2002).
- The New Zealand motor vehicle study attempts to apportion the measured and estimated PM10 concentrations and hence deaths to different sources (e.g. motor vehicles and domestic home heating).
5.1 The New Zealand motor vehicle impact study
The New Zealand motor vehicle impact study was a risk assessment carried out by Fisher et al (2002) to quantify the impact of PM10 concentrations from motor vehicles in New Zealand on mortality. The three main components of the study were:
- the relationship between PM10 concentrations and premature mortality
- estimates of exposure to PM10 concentrations in New Zealand
- the contribution of motor vehicle emissions to PM10 concentrations and exposure in New Zealand.
The methodology for assessing the impact of PM10 concentrations on mortality was taken from the Kunzli et al (2000) risk assessment for Austria, France and Switzerland. The studies use a relationship of a 4.3% increase in mortality for every 10 µgm-3 increase in annual average PM10 concentrations above 7.5 µgm-3 and are based on the longitudinal cohort studies of Dockery et al (1993) and Pope et al (1995). This impact estimate is about 4-5 times higher than the relationships typically observed in the standard times series epidemiological studies. This underestimate is attributed to the time-series methodology, which associates only those deaths that occur a relatively short time after the pollution episode to PM10 concentrations. Thus they are limited to a selection of the acute impacts but do not estimate the reduced life expectancy due to long-term morbidity enhanced by air pollution (Kunzli et al, 2000).
Estimates of exposure to PM10 concentrations for the New Zealand motor vehicle impacts study were based on measured PM10 concentrations, where available and estimated exposure in other areas. The study was limited to areas containing at least 500 people per square kilometre, which included about 80% of the New Zealand population. In some of the larger cities, dispersion modelling was combined with monitored concentrations to estimate exposure. The main limitations in estimating exposure is lack of monitoring data in some areas as well as uncertainties associated with the accuracy of modelling in predicting exposure in the larger cities. The uncertainties associated with lack of monitoring data are unlikely to be significant owing to the relatively small number of people residing in the locations where data are lacking.
The third component of the New Zealand motor vehicles impact study was the attribution of measured or estimated PM10 concentrations to motor vehicle sources. This was generally based on emission inventory studies where available and general estimates based on population, or similarities between locations in the absence of such studies. Uncertainties include absence of the quantification of natural sources including sea spray and dusts in many of the inventories and broad assumptions on similarities between areas.
Table 5.1 presents estimates of mortality for people over 30 exposed to PM10 concentrations from all sources and just from motor vehicles. This data is based on all towns and cities with a population of greater than 5000 people, encompassing 78% of the population, using annual average PM10 data for an average year assuming a no effects threshold of 7.5 µgm-3.
Table 5.1: Mortality estimates for PM10 in New Zealand
| Total mortality* | Vehicle related mortality* | |
|---|---|---|
|
Auckland |
436 |
253 |
|
Wellington |
79 |
56 |
|
Christchurch |
182 |
41 |
|
Dunedin |
48 |
6 |
|
Rest of North Island |
133 |
21 |
|
Rest of South Island |
80 |
19 |
|
Total New Zealand |
970** |
399** |
Source: Fisher et al (2002).
* Based on a 4.3% increase in mortality, in population over 30, per 10 µgm-3 increase in annual average PM2.5 concentrations above 7.5 µgm-3.
** Note: Some differences in totals occur as a result of rounding in calculations.
A number of limitations with the New Zealand motor vehicle impact study identified by the reviewers are presented as an Appendix to the Fisher et al (2002) report. These include focus on a number of uncertainties associated with the estimates of impact including issues of population exposure as well as apportionment of sources to motor vehicles and other sources. There are clearly a number of uncertainties, particularly with the latter apportionment assessment. However, these are acknowledged in the report and further studies to improve on these data and subsequent impact estimates will be undertaken using funding provided by the Health Research Council, the Ministry of Transport and the Ministry for the Environment.
5.2 Risk assessment for Nelson
A risk assessment of the impact of PM10 concentrations in Nelson was estimated based on the number of 10 µgm-3 increments in 24-hour average PM10 concentrations measured in Nelson, the baseline annual mortality and morbidity data for Nelson based on the years 1997 and 1998, and the dose-response relationships from epidemiological studies (Wilton, 2001).
The baseline mortality and morbidity data for Nelson were obtained from the New Zealand Heath Institute Statistics (NZHIS). Data were not available for years beyond 1998 so statistics for 1997 and 1998 were used. Health statistics data for dose-response relationships observed in Christchurch were used in preference to the WHO statistics because of the similarities in particle sources between Christchurch and Nelson.
The assessment was based on the following information and assumptions:
- a 1% increase in all cause mortality for every 10 µgm-3 increase in 24-hour average ambient PM10 concentrations (as reported in Hales, 1999)
- a 3% increase in respiratory mortality for every 10 µgm-3 increase in 24-hour average ambient PM10 concentrations (as reported in Hales, 1999)
- a 2.3% increase in respiratory admissions (including asthma) per 10 µgm-3 increase in 24-hour average PM10 concentrations (as reported in McGowan et al, 2000)
- a 1.25% increase in asthma admissions per 10 µgm-3 increase in 24-hour average PM10 concentrations (as reported in McGowan et al, 2000)
- a 0.85% increase in cardiac admissions per 10 µgm-3 increase in 24-hour average PM10 concentrations (as reported in McGowan et al, 2000)
- no threshold exists below which effects do not occur
- a total of 830 10 µgm-3 increments in Nelson based on estimated average exposure, per year
- baseline mortality and morbidity data from 1997 and 1998
- classification of illnesses as per the International Classification Codes described in appendices one and two
- the study area for the assessment including the following Nelson Census Area Units: Clifton, Atawhai, The Wood, Britannia Heights, Trafalgar, Kirks, Bronte, Atmore, Tahunanui, Tahuna Hills, Toi Toi, Broads, Grampians, The Brook, Nelson Airport, Nayland, Waimea Inlet East, Saxton, Langbein, Maitlands, Isel Park, Enner Glynn and Ngawhatu.
The risk assessment indicates that around eight deaths per year are likely to occur as a result of existing particle concentrations. In addition, six respiratory hospital admissions are likely to occur each year as a result of PM10 concentrations, including one to two asthma admissions. Around eight cardiac admissions could be attributed to particle concentrations, giving a total of around 14 hospitalisations per year that are likely to occur as a result of existing particle concentrations. These estimates include only a selection of acute health impacts, namely those that would occur near to the pollution event. The analysis is therefore highly conservative and estimates of both mortality and hospitalisations are likely to underestimate actual impacts. Because of the assumptions associated with the analyses these data should be treated as indicative of order of magnitude only for the minimum impact.
The assessment also estimates the number of PM10 related restricted activity days (RAD), which are defined by Ostro (1987) as days spent in bed, days missed from work and days when activities are partially restricted due to illness. Ostro examined the association between PM2.5 concentrations and RAD in the adult population during a two-week period in 49 United States metropolitan areas. In an assessment of the costs associated with concentrations of particles, the American Lung Association extrapolated the results of the Ostro study to population-based statistic. They indicated an increase in RAD of approximately 91,200 RAD each year per million of population for every 1 µgm-3 increase in annual average PM2.5. Assuming a ratio of PM2.5 to PM10 of 0.6:1 and an annual average PM10 concentration of 26 µgm-3, these data indicate that around 58,000 RAD could be expected to occur each year as a result of concentrations of particles in Nelson.
5.3 Risk assessment for Christchurch
A similar risk assessment was carried out for Christchurch in 1999, based on health statistics for 1992 and 1993, air quality monitoring data from a central monitoring site and the relationships between PM10 and mortality and morbidity from the Hales (1999) and McGowan (2002) studies. These include a 1% increase in all cause mortality for every 10 µgm-3 increase in 24-hour average ambient PM10 concentrations, a 3% increase in respiratory mortality for every 10 µgm-3 increase in 24-hour average ambient PM10 concentrations, a 2.3% increase in respiratory admissions (including asthma) per 10 µgm-3 increase in PM10 concentrations, a 1.25% increase in asthma admissions per 10 µgm-3 increase in PM10 concentrations and a 0.85% increase in cardiac admissions per 10 µgm-3 increase in 24-hour average PM10 concentrations.
The assessment indicates that existing PM10 concentrations are associated with around 40-70 deaths per year and around 75-100 hospitalisations (Wilton, 1999). An estimate of restricted activity days was also carried out based on the American Lung Association's extrapolation of the Ostro (1987) paper. Results indicated around 300,000 to 600,000 days per year when people's activities are restricted as a result of particle pollution (Wilton, 2001).
