Medway is the most polluted part of the county, says Friends of the Earth.
The environmental organisation has analysed data on air quality across the country and found large areas of Strood, Rochester, Chatham and Gillingham suffered with much higher levels of pollution than anywhere else in Kent and the south east.
As part of the study, England was divided into neighbourhoods, each with an average population of 1,700 people.
The amount of nitrogen dioxide (NO2) and particulate matter (PM2.5) – the two deadliest air pollutants – was calculated for every area.
In Medway, 43 of its neighbourhoods had pollution that was at least double World Health Organisation recommended levels. Next worst in Kent was Gravesham, with six, and Dartford, with three.
FoE air pollution campaigner Jenny Bates said: “It’s a scandal that people in so many parts of the south east live in areas where average air pollution levels are twice World Health Organisation guidelines for either or both of two of the deadliest pollutants.
"Road traffic is the biggest threat to clean air and the major source of the toxic gas nitrogen dioxide in the air we breathe.
"Air pollution hits the most vulnerable the hardest, particularly children who can grow up with reduced lung function if living in an area with poor air quality, but also the elderly who are more likely to have a pre-existing condition worsened by bad air."
Dr Julian Spinks, GP and director at Medway Practices Alliance, said the main reason Medway had so much pollution was because it is a major urban area with congested roads.
He said: "A lot of the traffic that's going through the Medway towns is confined to the A2 corridor.
"Stationary traffic is far worse than moving traffic, because you actually pack the cars in tighter so there's more cars producing pollution in a particular mile.
"When I go through the Medway towns I spend a lot of my time waiting in queues and at traffic lights.
"It's hard to see how to tackle that without reducing things like levels of traffic, and trying to get the flow of traffic better through Medway."
Dr Spinks said the tiny particles from vehicles' exhausts – a far cry from the 1950s'-style smog – can get past your nose and mouth and end up in your lungs.
"They're absolutely minute," he added. "You aren't going to see it floating around. It can trigger asthma, chronic lung disease, it also affects your heart and can give you heart disease and even stroke."
Medway Council measures air quality and says there are four areas where the levels of nitrogen dioxide are too high: High Street, Rainham; Pier Road, Gillingham; Four Elms Hill, Chattenden and Central Medway – which stretches from London Road, Strood to Chatham Hill.
The authority, which says it is committed to improving the air quality for its residents and future generations, has an Air Quality Action Plan setting out a variety of measures to manage emissions from vehicles.
It includes proposals for managing traffic, improving vehicle efficiency, promoting alternative methods of transport and walking and cycling, and the use of low-emission vehicles.
More information on air quality in Medway can be found by clicking here to read the authority's latest Annual Status Report.
According to Public Health England, air pollution causes between 28,000 and 36,000 premature deaths a year.
The FoE says government targets for cutting the amount of air pollutants by 2040 "must be improved" and should be met by 2030.
It also warns the problem could be worse than the study suggested, because the figures are based on the latest available information, from 2020, when lockdown measures meant pollution levels were lower.
Dr Spinks added: "We have seen a reduction of pollution over the last 20 or 30 years, but there is a bit of a flattening out and possibly an uptick again.
"A few months ago I would have been a bit optimistic because we were moving towards renewables and so on, but ironically the problems in Ukraine and the difficulty of maintaining things like the generation of electricity is pushing us back towards fossil fuels, which is not a good idea.
"Also it seems to be that as public spending is going to be hit, public transport is being less used and people are going back to cars.
"We could make green roads and pathways through towns and villages so people can walk or cycle away from pollution. But we will have to allow for the fact that we rely on some transport."