Air pollution is nearly twice the legal limit in parts of Brentford, a new survey has found.

Levels of the toxic gas nitrogen dioxide (NO2) beside the A4 and M4 are nearly twice the EU threshold of 40 micrograms per cubic meter (µg/m3), tests carried out this summer showed.

A total of 36 air quality monitors were placed around Brentford using funding from Mapping for Change, a citizens' science project based at University College London (UCL).

Hounslow Green Party, which installed the devices, said the average NO2 reading was 49.07 µg/m3 - well in excess of the legally-binding EU limit.

The highest figure recorded was 73.19 µg/m3, taken beside the A4 close to the junction with Boston Manor Road.

Hounslow Green Party press officer Nico Fekete said: "We are aware air monitoring is costly for taxpayers and London councils and only serves to confirm what we already know: emissions are too high.

"Add to this EU fines, NHS costs and 9,400 deaths related to air pollution and we can only conclude air pollution is not a distant threat and we need to clean-up our air today."

A map showing some of the sites monitored around Brentford. Green dots indicate NO2 levels below 40 µg/m3, light orange 40-60 and dark orange 60-80

Hounslow Green Party said the measurements were taken over the last week of the summer term and first three weeks of the school holidays when traffic is generally lighter, meaning the true figure could be even higher.

It says the findings support the case for the introduction of a 20mph speed limit on all but Hounslow's arterial roads, a move about which the council has been consulting residents.

Air pollution contributed to the deaths of 9,400 people in London in 2010, according to a study by scientists at King's College London, published this summer. Of those casualties, 5,900 were attributed to long-term exposure to NO2.

Louise Francis, chief executive of Mapping for Change, said: "NO2 is a noxious substance and has been associated with a number cardiovascular diseases.

"Research suggests that different driver behaviours, such as braking and acceleration can influence local air quality. In 20 mph speed zones brake and tyre wear, which contributes to particulate pollution, is significantly reduced."

You can view the full findings of the study here.