INTRODUCTION
In both developed and rapidly industrialising countries, the major historic air pollution problem has typically been high levels of smoke and sulphur dioixide arising from the combustion of sulphur-containing fossil fuels such as coal for domestic and industrial purpose. The major threat to clean air is now posed by traffic emissions. Petrol and diesel-engined motor vehicles emit a wide variety of pollutants, principally carbon monoxide (CO), oxides of nitrogen (NOx), volatile organic compounds (VOCs) and particulates (PM₁₀), which have an increasing impact on urban air quality. In addition, photochemical reactions resulting from the action of sunlight on nitrogen dioxide (NO₂) and VOCs from vehicles leads to the formation of ozone, a secondary long-range pollutant, which impacts in rural areas often far from the original emission site. Acid rain is another long-range pollutant influenced by vehicle NOx emissions. In all except worst-case situations, industrial and domestic pollutant sources, together with their impact on air quality, tend to be steady-state or improving over time. However, traffic pollution problems are worsening world-wide. Below is an introduction to the principal pollutants produced by industrial, domestic and traffic sources:

NITROGEN DIOXIDE (NO2)
Nitric oxide (NO) is mainly derived from road transport emissions and other combustion processes such as the electricity supply industry. NO is not considered to be harmful to health. However, once released to the atmosphere, NO is usually very rapidly oxidised to nitrogen dioxide (NO₂), which is harmful to health. NO₂ and NO are both oxides of nitrogen and together are referred to as nitrogen oxides (NO_x).Nitrogen dioxide can irritate the lungs and lower resistance to respiratory infections such as influenza. Continued or frequent exposure to concentrations that are typically much higher than those normally found in the ambient air may cause increased incidence of acute respiratory illness in children.

PARTICLES (PM10)
Fine Particles are composed of a wide range of materials arising from a variety of sources including:
combustion sources (mainly road traffic);
secondary particles, mainly sulphate and nitrate formed by chemical reactions in the atmosphere, and often transported from far across Europe;
coarse particles, suspended soils and dusts (e.g. from the Sahara), seasalt, biological particles and particles from construction work. Particles are measured in a number of different size fractions according to their mean aerodynamic diameter. Most monitoring is currently focussed on PM₁₀, but the finer fractions such as PM_2.5 and PM1 are becoming of increasing interest in terms of health effects. Fine particles can be carried deep into the lungs where they can cause inflammation and a worsening of the condition of people with heart and lung diseases. In addition, they may carry surface-absorbed carcinogenic compounds into the lungs.

Air quality is measured by comparing against a range of health-effects based standards. This page gives information about what this means.

Air Quality Standards and Objectives

Standards for air pollution are concentrations over a given time period that are considered to be acceptable in the light of what is scientifically known about the effects of each pollutant on health and on the environment. They can also be used as a benchmark to see if air pollution is getting better or worse.

An exceedence of a standard is a period of time (which is defined in each standard) where the concentration is higher than that set down by the standard. In order to make useful comparisons between pollutants, for which the standards may be expressed in terms of different averaging times, the number of days on which an exceedence has been recorded is often reported.

An objective is the target date on which exceedences of a standard must not exceed a specified number.

The objectives adopted in the UK are defined in the latest Air Quality Strategy for England, Scotland, Wales and Northern Ireland, published on 17th July 2007. Those which are limit values required by EU Daughter Directives on Air Quality have been transposed into UK law through the Air Quality Standards Regulations 2007 which came into force on 15th February 2007.

A summary of the current UK Air Quality Objectives is provided here.

* not included in regulations at present
      Shaded data shows new objectives

* not included in regulations at present
⁺ AOT 40 is the sum of the differences between hourly concentrations greater han 80 µg m^-3 (=40ppb) and 80 µg m^-3, over a given period using only the 1-hour averages measured between 0800 and 2000.
      Shaded data shows new objectives

Air Pollution Information Service

Index and Bands

In the UK most air pollution information services use the index and banding system approved by the Committee on Medical Effects of Air Pollution Episodes (COMEAP). The system uses 1-10 index divided into four bands to provide more detail about air pollution levels in a simple way, similar to the sun index or pollen index.

• 1-3 (Low)
• 4-6 (Moderate)
• 7-9 (High)
• 10 (Very High)

The overall air pollution index for a site or region is calculated from the highest concentration of five pollutants:

• Nitrogen Dioxide
• Sulphur Dioxide
• Ozone
• Carbon Monoxide
• Particles < 10µm (PM10)

Air Pollution Forecasts

Air Quality Forecasts are issued on a regional basis for three different area types:

• In towns and cities near busy roads
• Elsewhere in towns and cities
• In rural areas

Forecasts are based on the prediction of air pollution index for the worst-case of the five pollutants listed above, for each region.

Health Advice

Latest studies report that:

• When air pollution is LOW (1-3) effects are unlikely to be noticed even by those who are sensitive to air pollution.
• When air pollution is MODERATE (4-6) sensitive people may notice mild effects but these are unlikely to need action.
• When air pollution is HIGH (7-9) sensitive people may notice significant effects and may need to take action.
• When air pollution is VERY HIGH (10) effects on sensitive people, described for HIGH pollution, may worsen.

Air pollution can cause short-term health effects to sensitive individuals (people who suffer from heart disease or lung diseases, including asthma). Effects on sensitive people can be reduced by spending less time outdoors. 'Reliever' inhalers should lessen effects on asthma sufferers.

More details on effects, including long-term, are available in a free leaflet 'Air Pollution - what it means for your health'.

Air Pollution Bandings and Index and the Impact on the health of People who are Sensitive to Air Pollution

Boundaries Between Index Points for Each Pollutant

* For ozone, the maximum of the 8 hourly and hourly mean is used to calculate the index value.