Under the Microscope: People have long been concerned about the quality of the air they breathe outdoors, writes Prof William Reville.
In 1273 the use of coal was banned in London because smoke is harmful to health. Traditionally the principal air pollution problem was smoke and sulphur dioxide arising from the burning of sulphur-containing fossil fuels, principally coal, for industrial purposes. Traffic emissions now pose the main threat to clean air. Air quality is of particular concern to people who suffer from heart conditions or lung problems such as asthma or bronchitis.
The Environmental Protection Agency (EPA) co-ordinates air monitoring in Ireland in accordance with EU directives. Monitoring stations are operated by the EPA and by local authorities. Further details at www.epa.ie/ourenvironment/air.
Ireland does not have serious outdoor air problems. A wide variety of pollutants are emitted by petrol and diesel engines, mainly carbon monoxide (CO), oxides of nitrogen (NOx), volatile organic compounds (VOCs) and particulate matter (PM). Also, sunlight can act on nitrogen dioxide (NO2) and VOCs to produce ozone (O3), a secondary pollutant that can impact on sites far from the original emission site. In general, the effects of industrial and domestic emissions on air quality are slowly lessening over time, but traffic pollution is worsening worldwide.
Why does burning petrol in engines cause air pollution? Petrol is composed almost entirely of a mixture of chemical compounds called hydrocarbons, compounds that always contain two key elements, hydrogen (H) and carbon (C). The burning of petrol is a chemical reaction between oxygen (O2) in air and the hydrocarbon molecules. O2 combines with the H to produce water (H2O) and also with C to form carbon dioxide (CO2). Neither H2O nor CO2 have any implications for human health, although CO2 does enhance the greenhouse effect.
So, where does the petrol-induced air pollution come from? There are three main sources: (a) engines don't burn the petrol completely; (b) the petrol is not pure and may have several additives; (c) the high engine temperature and pressure induce nitrogen (N2) and O2 in air combine together to produce NOx.
I will now run through the main pollutants that lower the quality of air. CO is a colourless, odourless gas resulting from incomplete burning of fuel in engines. It combines with haemoglobin in blood and lowers its capacity for carrying oxygen, thereby straining the heart. It can be present in air in concentrations ranging from 10-600 micrograms per cubic metre. The EU limit is 10 milligrams per cubic metre for exposure over an eight-hour period (averaged). The origin of NOx has already been explained. In moist air NOx reacts with water to form nitric acid (HNO3) and this can exacerbate the symptoms of asthma. NOx is normally present in air at 19-85 micrograms per metre cubed, but can rise as high as 750 micrograms per metre cubed in peak traffic. The EU limit for NO2 is 200 micrograms per metre cubed over one hour.
Burning fuel that contains sulphur releases SO2 which oxidises in moist air to sulphuric acid (H2SO4). It can cause wheezing and bronchitis. SO2 concentration is usually below 90 micrograms per metre cubed in urban areas. The EU limit is 350 micrograms per metre cubed over one hour.
Sunlight interacts with polluted air that contains NOx to form O3 (ozone). Ozone is highly reactive, makes asthma worse and irritates the nose, eyes and throat. Levels of O3 are normally below 40 micrograms per metre cubed but can rise to 200 micrograms per metre cubed in summer. The EU limit is 90 parts per billion over one hour.
Particulate material (PM) is a general term that covers a range of particle sizes from coarser to finer particles, including soot from diesel engines. SO2 and NOx react to form fine particles. Particles smaller in size than 10 micrometres (millionths of a metre) are abbreviated PM10 and can penetrate and lodge deep in the lung. PM10 are mainly produced in cities by diesel engines. The EU limit has been set at 50 micrograms per metre cubed from 2005.
Volatile organic compounds (VOCs) are mainly hydrocarbons that arise from evaporation of fuel. The principal VOC is methane from natural gas. Benzene makes up about 2 per cent of petrol and can also be made in engines during fuel burning. Benzene is a known cancer-causing agent. The EU annual limit is five micrograms per metre cubed.
Most of us do much of our driving around town, making short journeys in petrol-driven cars in which the driver is the sole occupant. It would be much better for air quality if we all used special cars for city driving - little cars driven either by electric motors or by hybrid electric/petrol motors, basically about the size of large motorbikes with a car-like protective superstructure.
Eureka is a pan-European network for market-orientated industrial research and development and has just announced the launch of the Minimobil, specifically designed for congested cities. It uses a hybrid drive, combining the environmental benefits of an electric motor with the range of a petrol engine. It measures one metre by two metres and four Minimobils can fit into a standard parking space. It is designed for personal city transport. It can travel up to 50km by battery alone and up to 350km using its low-emission petrol engine. Energy consumption and pollutant emissions are much lower in a Minimobil.