The main air pollutants considered in the UK are benzene (C₆H₆), 1-3, butadiene (C₄H₆), carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO₂), particulate matter (PM₁₀ and PM_2.5), sulphur dioxide (SO₂), ozone (O₃) and polycyclic aromatic hydrocarbons (PAHs).  The Government’s policy on air quality is set out in the Air Quality Strategy (AQS). This sets standards and objectives for these ten main air pollutants to protect human health, vegetation and ecosystems. 

The air quality standards are long-term benchmarks for ambient pollutant concentrations which represent negligible or zero risk to health, based on medical and scientific evidence.  These are general concentration limits, above which sensitive members of the public (e.g. children, the elderly and the unwell) might experience adverse health effects.

The air quality objectives are medium-term policy-based targets set by the Government which take into account economic efficiency, practicability, technical feasibility and timescale.  Some objectives involve a margin of tolerance, i.e. a limited number of permitted exceedances of the standard over a given period.

Of these ten pollutants the key pollutants of concern are NO₂, PM₁₀ and PM_2.5 as these are the closest to the objective levels and are produced from road traffic which is the key source of pollution in the majority of places. 

The objective levels set out in the AQS for these pollutants are:

Pollutant	Averaging Period	Objective Level
Nitrogen Dioxide (NO2)	Annual Mean	40µg/m3
	Hourly Mean	200µg/m3 not to be exceeded more than 18 times per year
Particulate Matter not exceeding 10um in diameter (PM10)	Annual Mean	40µg/m3
	24-hour Mean	50µg/m3 not to be exceeded more than 35 times per year
Particulate Matter no exceeding 2.5um in diameter (PM2.5)	Annual Mean	25µg/m3
Note: PM2.5 objective not yet incorporated into UK law, however, should still be considered.

Nitrogen Dioxide (NO2)
Combustion processes in air form oxides of nitrogen (NOx) which comprises mainly NO and a small proportion of NO2. NO is a relatively innocuous compound, but it rapidly transforms to NO2 by reaction with ozone.
NO2 is associated with adverse effects on human health. High levels of NOx can also have an adverse effect on vegetation including leaf damage and reduced growth. Deposition of pollutants derived from NOx contribute to eutrophication and acidification effects.

Particulate Matter (PM10 and PM2.5)
Particulate Matter is generally categorised based on the size of particles i.e. PM2.5 is particles with a diameter of less than 2.5um. Particulate matter is made up of a wide range of materials and arise from a variety of sources both human-made and natural. Road transport gives rise to particles from engine emissions, tyre and brake wear and other non-exhaust emissions.
Exposure to particulate matter is associated with respiratory and cardiovascular illness. PM10 particles are likely to be inhaled into the thoracic region of the lung. PM2.5 which makes up around two thirds of PM10 emissions and concentrations has a stronger association will ill-health effects.

Source of air pollutants arising from development
During construction of a development, there will be emissions of dust and particulate matter to air from on-site activities and emissions of pollutants from the exhaust of plant equipment and road vehicles.
During operation, the key emissions to air are likely to be emissions of pollutants from the exhaust of road vehicles and energy generating plant. Developments that incorporate specific industrial processes may have additional emissions to air.