Dr Ravi Gooneratne
30 May 2005
As the city slides deeper into the trough of another Christchurch winter, more than 350,000 people face the prospect of breathing air which will at times exceed WHO guidelines and cause further stress for people with respiratory illnesses.
According to an Otago University Study up to 70 people in Christchurch will die this year from respiratory illnesses and heart problems linked to air pollution. But if this study was based on the new smog measuring system, the figure would have been much higher.
After years of debate, city authorities are starting to make progress with Christchurch’s winter problems. To date our efforts to improve air quality have centred on the small particulates (PM10), which work their way deep into lungs and the smallest airways. Small particulates inflame the cells lining the lungs and release chemicals which can induce a heart attack.
More than 90 per cent of these pollutants come from burning wood and coal in open fires and in low-technology burners. Tougher rules for home heating devices and promotion of clean energy alternatives, partly through financial incentives, are achieving a gradual conversion.
Clean air advocates were looking forward to further gains this year through long-promised plans for mandatory testing of vehicle emissions but, in a surprise move, the Government has shelved these plans due to concerns over the costs and accuracy of a testing regime.
Critics have good reason to be disappointed. Suspended particulates may be our most visible form of air pollution, but they are certainly not the only material that presents a health risk and an environmental impact.
Unlike Auckland, motor vehicles in Christchurch contribute only about 5% of particulate pollution but they still add to the overall air pollution problems with carbon monoxide, volatile organic compounds (due to partly burnt fuel), sulphur oxides and nitrogen oxides. We cannot see these chemicals, but they are contaminants which affect the environment and human health.
So, although emissions from individual cars are generally low relative to the smokestack pollution from burning solid fuels in winter, driving a motor car is probably our most common polluting activity. Older cars pollute more because of the incomplete burning of hydrocarbons compared to new cars which have more efficient exhaust emission controls.
The health risk from vehicle emissions depends on the chemicals we are exposed to, the frequency of exposure, and individual factors such as age, pre-existing medical conditions and pregnancy.
Even a properly maintained petrol passenger which travels 20,000km a year with average fuel economy will emit about 35 kg of hydrocarbons (which include benzene, a cancer causing chemical), 250 kg of carbon monoxide, 17 kg of nitrogen oxides and over 5000 kg of carbon dioxide. Compared to petrol engines, diesel engines produce lesser amounts of unburnt and partially burnt fuel (therefore less hydrocarbons and carbon monoxide) but produce more nitrogen oxides and sulphur as oxides, smoke, and fine particles.
Carbon monoxide can interfere with transport of oxygen in blood to tissues including the brain and the heart. Therefore, even at low levels, it can cause tiredness, and in people with chronic heart disease, chest pains. At higher concentrations, it can cause dizziness, weakness, nausea, confusion and disorientation. Pregnant women and their unborn babies, infants, and elderly are particularly sensitive. Carbon dioxide does not directly affect human health but it is a “greenhouse gas” that traps the earth’s heat and contributes to the potential for global warming.
Hydrocarbons and nitrogen oxides are the second and third largest air pollutants in Christchurch, with emissions around 9,000 and 7,000 tonnes per year respectively. Winter smog in Christchurch occurs when light wind, low cloud cover and temperature inversion occurs with the trapping of airborne particulates and other pollutants in the lower atmosphere. Hydrocarbons can react in sunlight with oxides of nitrogen, volatile organics and oxygen under conditions of photochemical smog to form ozone. Prolonged inhalation of ozone can damage lung tissue and weaken the immune system.
The big nasty in our vehicle emissions, in terms of human health, is benzene. In any country, petrol is the number one source of benzene exposure. Many countries have already legislated an upper limit for benzene levels (per cubic metre of air) due to the sheer density of vehicles. This is where emission testing has real merit.
Only about 40 percent of the benzene originally present in the fuel is emitted in exhaust gases. More than 50 percent of benzene in the emissions is formed during combustion in the engine. It has to be measured at the tailpipe to get an accurate estimate of its contribution to air pollution, and the most practical way to achieve this is through the mandatory testing that is carried out in other developed countries such as Australia.
The concentration of benzene in Christchurch air is in general less than 5 micrograms per cubic metre of air, well below the levels in urban European urban areas of 35-100, but it is now accepted that environmental measurements of benzene concentrations are poor predictors of individual exposure. This is because most people spend 80% of their time indoors where concentration of benzene can be quite high through off-gassing from building materials and some types of paints and, importantly, tobacco smoke. Individual cigarettes can contain up to 30 micrograms of benzene, making smoking the single most significant source of benzene exposure. The risk from second hand smoke was one of the key factors in new regulations to provide clean air in workplaces, restaurants, cafes and bars.
Overall, air pollution levels are slowly declining due to the impact of control programmes such as the unleaded fuel strategy introduced in 1996. Further health gains could be made through a vehicle testing programme running alongside the new national air standards and other measures such as penalties for removing anti-pollution devices such as catalytic converters from vehicles, promoting alternatives to driving and providing incentives to run efficient, low emission vehicles.
Vehicle emission standards and a practical testing regime should be possible in a country that has much to gain from a good environmental reputation, and never-ending pressure on the health system.
When used in conjunction with new traffic management technologies and more sensible use of privately owned cars, emissions testing could make a significant contribution to the air that we all share.
Dr Ravi Gooneratne is an Associate Professor in Toxicology at Lincoln University.