Those most at risk include street cleaners and parking staff. The study measured average levels of nitrogen dioxide in the air using data from 110 monitoring stations.
Air pollution can aggravate health problems like asthma and increases the likelihood of lung disease. It contibuted to more than 28,000 deaths across the UK in 2010, according to recent government figures.
The study recorded unsafe levels of air pollution in several parts of London, as well as in Cambridge, Aberdeen, Bath, Glasgow, Leeds, Manchester, Birmingham and Chepstow.
The union said councils should do more to identify the areas and times of day when the problem was worse and then inform local people.
It also said street cleaning and refuse collection routes should be changed to avoid times when air pollution was very high.
The European Union sets out guidelines for nitrogen dioxide (NO2) levels which state that they should not exceed 40µg/m3 on average over the course of a year.
John McClean, national officer of the GMB, said the figures from the study confirmed “the urgent need for better air quality monitoring in urban and built-up areas”.
“Clean air should be a right, not a privilege,” he said.
He also stressed that road transport was a major problem, adding: “We reiterate our call for high-polluting vehicles to be banned from city centres, and for local authorities to take immediate action in priority areas such as near schools, hospitals and doctors’ surgeries.”
In February, the European Commission launched legal proceedings against the UK for failing to deal with air pollution, saying that levels of NO2 – mainly from diesel engines – were “excessive” in many British cities.
DEFRA has previously said it is a “challenge” to meet air pollution targets near busy roads, but that air quality has “improved significantly” in recent decades.
In Southampton in late June, campaigners called for action over fears that air pollution in the city is not being properly monitored, following the closure of two monitoring stations, at Redbridge and Bitterne.