Community groups recording air pollution in London found eight out of nine areas surveyed breached EU limits for nitrogen dioxide (NO2) in the air.
The University College London (UCL) project found the highest level of NO2 in the areas was in Marylebone in central London.
Diesel vehicles are a key source of NO2, which is linked to a range of respiratory illnesses.
London's deputy mayor for environment and energy said NO2 was a "problem".
The results are part of the social enterprise Mapping for Change's Air Quality Monitoring project.
Community groups were given kits to measure the amount of NO2 in July in areas including Marylebone, Soho, Walthamstow, Brentford, Haringey and Ham.
The highest reading was on Marylebone Road, in central London, measuring 145 micrograms of NO2 per cubic meter air (µg/m3), which is nearly four times the EU legal limit of 40µg/m3.
This was followed by Shaftesbury Avenue, in Soho, at 119µg/m3.
Tests in suburban areas like Walthamstow and Brentford also showed nearly double the EU limit.
Louise Francis, from UCL and co-founder of Mapping for Change, said the results were a snapshot of the level of pollution in the capital but the actual levels of NO2 could be higher as holiday levels were often lower than other times of the year.
She said that the public could make changes to reduce their intake of NO2 by walking down side roads instead of major routes.
Matthew Pencharz, deputy mayor for environment and energy, said: "NO2 remains a problem. We have now seen a 12% reduction in measured NO2 across London showing we are making real progress."
He added that the introduction of taxi age limits and cleaner buses had also improved air quality, as would the introduction in 2020 of the Ultra Low Emission Zone (ULEZ), which will only allow zero and low-emission vehicles in central London.
Public Health England suggests about 3,000 people each year are estimated to die from polluted air in the capital, making it London's second highest cause of death after smoking.