The main cause is a cocktail of pollutants from petrol and diesel-powered vehicles producing carbon monoxide, oxides of nitrogen, and fine particle matter that damage health.
Medical experts estimate that traffic pollution causes up to 40,000 deaths a year and rising.
The 1995 Environment Act and 2008 EU Air Quality Directive requires local authorities to monitor air pollution against national guidelines and take action when they are exceeded.
Local councils are also required to publish information on air quality within their area.
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The UK, along with other EU members, is failing to take action against nitrogen oxide in breach of Article 13 of the EU Directive.
Last year the government was taken to court by activist lawyers Client Earth for non-compliance and, on appeal, the Supreme Court found in their favour.
The government’s response is to designate five clean air zone cities which include Southampton in the south.
Portsmouth and Southampton breach World Health Organisation guidelines for 10 micrometer particles (one seventh of the width of a human hair)
Air quality is monitored locally through diffusion tubes located near the road in areas with heavy traffic movement.
Wherever the EU standard is exceeded, councils are required to declare an Air Quality Management Action Area (AQMA).
Portsmouth had 13 in 2010, of which five remain.
Havant’s levels have so far not justified an AQMA but some areas are running close to where one could soon be justified.
Elsewhere, Southampton has eight AQMAs, the highest in Hampshire, and Chichester has two.
Winchester city centre, situated in a valley, is one of the most polluted in Hampshire.
Improving air quality is one of the most challenging environmental challenges in our increasingly urbanised society.
Low emission zones and cleaner vehicles would be a huge step forward.
Electric vehicles may be cleaner provided they use green energy to re-charge.
More major investment is needed for providing, clean, affordable public transport and safer routes for cycling and walking.
Clean air is a basic human right, not a nice-to-have optional extra.