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Appendix 3: Vehicle Emissions and Effects

Transport activities have the potential to discharge a large number of different contaminants. Table A3.1 lists those identified by Environment Australia as typically discharged from motor vehicles (Environment Australia, 2000a).

Table A3.1: Substances present in motor vehicle emissions

Acetaldehyde

n-hexane

Acetone

Lead and compounds

Benzene

Manganese and compounds

1,3-butadiene

Nickel and compounds

Cadmium and compounds

Oxides of nitrogen

Carbon monoxide

Particulate matter ≤ 10 µm (PM10)

Chromium (III) compounds

Styrene

Cyclohexane

Polycyclic aromatic hydrocarbons (PAHs)

Chromium (VI) compounds

Sulphur dioxide

Cobalt and compounds

Toluene

Copper and compounds

Total volatile organic compounds (VOCs)

Ethylbenzene

Xylenes

Formaldehyde

Zinc and compounds

The health effects associated with some of the key emissions are briefly described below. Comprehensive information is available from a range of sources, including:

  • WHO (2005) Air Quality Guidelines: Global Update 2005, which includes a comprehensive review of health effects for particulate matter, ozone, nitrogen dioxide and sulphur dioxide

  • Ministry for the Environment, 2003a, Health Effects of CO, NO2, SO2, Ozone, Benzene and Benzo(a)pyrene in New Zealand

  • Ministry for the Environment, 2003b, Health Effects of PM10in New Zealand.

Carbon monoxide

Carbon monoxide is an odourless gas formed as a result of incomplete combustion of carbon-containing fuels, including petrol and diesel. Carbon monoxide is readily absorbed from the lungs into the bloodstream, which then reacts with haemoglobin molecules in the blood to form carboxyhaemoglobin. This reduces the oxygen-carrying capacity of blood, which in turn impairs oxygen release into tissue and adversely affects sensitive organs such as the brain and heart. Motor vehicles are the predominant sources of carbon monoxide in most urban areas.

As a consequence of the age of the vehicle fleet, New Zealand has relatively high urban air concentrations of carbon monoxide. Nearly 50% of the New Zealand car fleet is more than 10 years old, and only one in five vehicles is less than five years old. It is estimated that 71% of the vehicle fleet is fitted with catalytic converters. However, 10% of these were fitted before leaded petrol was phased out, and it is unlikely that these catalytic converters would still be effective (Ministry of Transport, 2006b).

Long-standing international (and national) ambient air quality standards/guidelines for carbon monoxide are based on keeping the carboxyhaemoglobin concentration in blood below a level of 2.5% to protect people from an increased risk due to heart attacks. This has led to little variation in the standards/guidelines, being typically 10 mg/m3 as an eight-hour average, and 30 mg/m3 as a one-hour average. That situation may soon change, because there is emerging research that indicates adverse health effects at carboxyhaemoglobin levels less than 2.5%. This new information is especially relevant to New Zealand, because of the relatively high urban air concentrations of carbon monoxide.

Nitrogen dioxide

Nitrogen oxides (primarily nitric oxide and lesser quantities of nitrogen dioxide) are gases formed by oxidation of nitrogen in air at high combustion temperatures. Motor vehicles are usually the major sources of nitrogen oxides in urban areas. Nitric oxide is oxidised to nitrogen dioxide in ambient air, which has a major role in atmospheric reactions that are associated with the formation of photochemical oxidants (such as ozone) and particulates (such as nitrates). Nitrogen dioxide is also a serious air pollutant in its own right. It contributes both to morbidity and mortality, especially in susceptible groups such as young children, asthmatics and those with chronic bronchitis and related conditions.

Nitrogen dioxide appears to exert its effects directly on the lung, leading to an inflammatory reaction on the surfaces of the lung. Ambient air quality standards/guidelines for nitrogen dioxide are set to minimise the occurrence of changes in lung function in susceptible groups. The lowest observed effect level in asthmatics for short-term exposures to nitrogen dioxide is about 400 µg/m3. Although fewer data are available, there is increasing evidence that longer-term exposure to about 80 µg/m3 during early and middle childhood can lead to the development of recurrent upper and lower respiratory tract symptoms. A safety factor of two is usually applied to these lowest-observed effect levels, giving air quality standards/guidelines for nitrogen dioxide of 200 µg/m3 as a one-hour average, and either 40 µg/m3 as an annual average or 100 µg/m3 as a 24-hour average.

Volatile organic compounds

Volatile organic compounds are a range of hydrocarbons, the most important of which are benzene, toluene, xylene, 1,3-butadiene, polycyclic aromatic hydrocarbons (PAHs), formaldehyde and acetaldehyde. The potential health impacts of these include carcinogenic and non-carcinogenic effects. According to the World Health Organisation, benzene and PAHs are definitely carcinogenic, 1,3-butadiene and formaldehyde are probably carcinogenic, and acetaldehyde is possibly carcinogenic. Non-carcinogenic effects of toluene and xylene include damage to the central nervous system and skin irritation. Heavier volatile organic compounds are also responsible for much of the odour associated with diesel exhaust emissions.

Motor vehicles are the predominant sources of volatile organic compounds in urban areas. Benzene, toluene, xylene and 1,3-butadiene are all largely associated with petrol vehicle emissions. The first three result from the benzene and aromatics contents of petrol, and 1,3-butadiene results from the olefins content. Evaporative emissions, as well as exhaust emissions, can also be significant, especially for benzene.

Motor vehicles are major sources of formaldehyde and acetaldehyde. These carbonyls are very reactive and are important in atmospheric reactions, being products of most photochemical reactions. PAHs arise from the incomplete combustion of fuels, including diesel.

Of the volatile organic compounds, the most important in the New Zealand context is benzene. The benzene content of petrol was dropped to 1% in 2006 and, as a result, ambient levels have fallen. Health effects data and standards/guidelines for hazardous air pollutants include recommended ambient air quality guidelines for benzene of 10 µg/m3 (now) and 3.6 µg/m3 (in 2010), both guidelines being annual average concentrations. The implied cancer risks (leukaemia) corresponding to those air concentrations are, respectively, 44 to 75 per million population and 16 to 27 per million population, based on World Health Organisation unit risk factors for benzene.

Sulphur dioxide

Sulphur oxides (primarily sulphur dioxide and lesser quantities of sulphur trioxide) are gases formed by the oxidation of sulphur contaminants in fuel on combustion. Sulphur dioxide is a potent respiratory irritant, and has been associated with increased hospital admissions for respiratory and cardiovascular disease, as well as mortality. Asthmatics are a particularly susceptible group. There appears to be a threshold concentration for adverse effects in asthmatics from short-term exposures to sulphur dioxide at a concentration of 570 µg/m3 for 15 minutes. Ambient air standards/guidelines are based on this figure; for example, the national ambient air quality standard is 350 µg/m3 as a one-hour average, and the national ambient air quality guideline is 120 µg/m3 as a 24-hour average. Sulphur dioxide concentrations in New Zealand are relatively low. However, WHO have significantly reduced their air quality guideline for sulphur dioxide in the 2005 review. At time of publication, the implications of this for New Zealand have not been reviewed.

Sulphur oxides from fuel combustion are further oxidised to solid sulphates, to a certain extent within the engine and completely in the atmosphere. The former inhibits the performance of exhaust emission control equipment for nitrogen oxides and particles, and this is a major reason why the sulphur content of petrol and diesel is being reduced internationally. Many countries are moving to 'sulphur-free' petrol and diesel (less than 10 ppm). It is an unfortunate reality that unless the sulphur content of fuel is less than about 120 ppm, vehicles with advanced emission control systems are actually net producers of additional PM10 because of oxidation of the sulphur oxides to sulphates.

Until recently, New Zealand had high-sulphur-content diesel (up to about 2,500 parts per million by volume). However, this was reduced to 50 ppm in 2006, and will be reduced to 10 ppm by 2009. The sulphur content of petrol will be reduced in 2008 from the current level of 150 ppm to 50 ppm. The Government has not introduced a date for the introduction of 10 ppm sulphur petrol.

Particulates

Particulate such as sulphates cause increased morbidity and mortality. The evidence on airborne PM and public health is consistent in showing adverse health effects at exposures experienced by urban populations in cities throughout the world, in both developed and developing countries. The range of effects is broad, affecting the respiratory and cardiovascular systems and extending to children and adults and to a number of large, susceptible groups within the general population. The risk for various outcomes has been shown to increase with exposure, and there is little evidence to suggest a threshold below which no adverse health effects would be anticipated. In fact, the lower range of concentrations at which adverse health effects has been demonstrated is not greatly above the background concentration. The epidemiological evidence shows adverse effects of particulates after both short- and long-term exposures.

Recent preliminary research is showing that it is probably the finer particles causing greater effects (PM2.5), and particles from diesel emissions possibly having greater effects than those from other sources.

Ozone

Ozone is a natural substance found in the atmosphere. At lower levels (in the troposphere, up to about 10 km altitude) it occurs through natural reaction with oxygen and is present in concentrations between about 30 and 60 µg/m3 depending on the latitude and season. At higher levels (in the stratosphere, above 10 km altitude) it forms an important barrier to dangerous ultraviolet light from the sun, and its loss, in the so-called 'ozone-hole', is detrimental to life on Earth. There is very little relationship between tropospheric and stratospheric ozone.

Ozone is also a secondary, urban air pollutant formed by reactions of nitrogen oxides and volatile organic compounds in the presence of sunlight. These primary emissions arise mainly from motor vehicles. Ozone is only one of a group of chemicals called photochemical oxidants (commonly called photochemical smog), but it is the predominant one. Also present in photochemical smog are formaldehyde, aldehydes and peroxyacetyl nitrate.

Ozone is another air pollutant that has respiratory tract impacts. Its toxicity occurs in a continuum in which higher concentrations, longer exposure and greater activity levels during exposure cause greater effects. It contributes both to morbidity and mortality, especially in susceptible groups such as those with asthma and chronic lung disease, healthy young adults undertaking active outdoor exercise over extended periods, and the elderly, especially those with cardiovascular disease. Substantial acute effects occur during exercise with one-hour exposures to ozone concentrations of 500 µg/m3 or higher.

Ozone, like particulate, is an air pollutant for which there is no indication of a threshold concentration for health effects. More than any other air pollutant there is considerable variation in air quality standards/guidelines for ozone because of the complexities involved in reducing ambient concentrations. In New Zealand a relatively pure approach has been taken and a national ambient air quality standard for ozone of 150 µg/m3 as a one-hour average, and a national ambient air quality guideline of 100 µg/m3 as an eight-hour average, have been established.