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 (PAH) |
|
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 described below.
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 percent of the New Zealand car fleet is more than 10 years old, and only one in five vehicles is less than five years old. Furthermore, only about one-quarter of the car fleet have catalytic converters, even though they have been mandatory in countries from where vehicles have been sourced since the 1970s.
Long-standing international (and New Zealand) air quality guidelines/standards for carbon monoxide are based on keeping the carboxyhaemoglobin concentration in blood below a level of 2.5 percent, in order to protect people from an increased risk due to heart attacks. This has led to little variation in the guidelines/standards, being typically 10 mg/m3, eight-hour average, and 30 mg/m3, 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 percent. 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 particles (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. Air quality guidelines/standards 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/m3during 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 guidelines/standards for nitrogen dioxide of 200 µg/m3, one-hour average, and either 40 µg/m3 annual average, or 100 µg/m3 24-hour average (these two longer-term exposure concentrations being roughly equivalent).
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 is high, often exceeding four percent by volume, especially for the "premium" grade, whereas many overseas countries restrict the benzene content to less than one percent by volume. Health effects data and guidelines/standards for hazardous air pollutants include recommended air quality guidelines for benzene of 10 µg/m3 (now) and 3.6 µg/m3 (when the benzene content of petrol is reduced), 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. Although sulphur dioxide concentrations in New Zealand are relatively low, and motor vehicles are minor contributors to ambient sulphur dioxide, the measured levels in Auckland (for example) have increased in recent years, after many years of decline, as a result of the increasing number of diesel vehicles (and the relatively high sulphur content of diesel in New Zealand). 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 guidelines/standards are based on this figure; for example, the guidelines for New Zealand are 350 µg/m3 one-hour average, and 120 µg/m3 24-hour average.
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 are being reduced internationally. New Zealand currently has a high-sulphur-content diesel (up to about 2,500 parts per million by volume), although reductions announced in 2002 should see this fall over the next few years, down to 50 ppm by 2007. 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 diesel is less than about 120 ppm, vehicles with advanced emission control systems are actually net producers of additional fine particles because of oxidation of the sulphur oxides to sulphates.
Particulates
Fine particles such as sulphates cause increased morbidity and mortality, and there are no apparent threshold concentrations for those health effects. As a result, the World Health Organisation has decided not to recommend air quality guidelines for particles, but most countries (including New Zealand) have been more pragmatic and have set guidelines (typically 50 µg/m3 for PM10, 24-hour average) aimed at minimising the occurrence of health effects. 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 particles, is an air pollutant for which there is no indication of a threshold concentration for health effects. (However, unlike particles, the WHO has established air quality guidelines for ozone.) More than any other air pollutant, there is considerable variation in air quality guidelines/standards for ozone, because of complexities involved in reducing ambient concentrations of it. In New Zealand a relatively "pure" approach has been taken, and air quality guidelines for ozone of 150 µg/m3, one-hour average, and 100 µg/m3, eight-hour average, have been established.
