Over-exposure to air pollution can be a health risk, so under the 1995 Environment Act the government introduced a national 'Air Quality Strategy', which all local authorities have to comply with. This involves carrying out reviews and assessments of the air quality in their area and comparing the results to air quality objectives.
Air quality objectives are health-based and so the focus of each review is the exposure of members of the public to potential excessive amounts of pollution to ensure they're not at risk.
Get an up-to-date air pollution report for the north west (external link)
Air quality links