There are calls for Wales to do more to cut air pollution and meet stricter World Health Organisation (WHO) limits.
It comes as 400 deaths a year were linked to poor air quality in south Wales, according to the Centre for Cities.
In Cardiff alone, 131 deaths in 2017 were related to levels of particles in the air, said the think tank.
The Welsh Government said it is committed to improving air quality, and is consulting on a new clean air plan.
Across Swansea and Neath Port Talbot, the pollution was linked to 158 deaths, while in Newport there were 113 deaths, Centre for Cities said.
"The deadly levels of polluted air in Wales are entirely legal. This needs to change," said the Centre for Cities chief executive Andrew Carter.
"Failure to act now will lead to more deaths in Wales."
The think tank said transport and burning fossil fuels were significant contributors to the levels of air pollution - known as particulate matter (PM).
Cities in south east England were singled out by the report for the highest levels of pollutants known as PM2.5, including London, Luton and Slough.
But Swansea and the surrounding area had the highest levels of PM2.5 for every 10,000 inhabitants - along with Belfast in Northern Ireland.
The city and surrounding region also had the highest levels of nitrogen dioxide pollution in the UK, along with Warrington in Cheshire and Middlesbrough.
Is air pollution really killing us?
There have been several scientific studies into both the short and long term impacts of air pollution on our health.
Reports for the UK's Parliamentary Committee on the Medical Effects of Air Pollutants (COMEAP) and the World Health Organisation have shown air quality can worsen asthma, lung function and increase hospital admissions, cardiovascular problems, and impact on death rates.
COMEAP again re-iterated its stance in 2018, stating: "There is good mechanistic evidence for a causal role of PM2.5 in shortening life."
Public Health England published research to examine just how PM2.5 pollutants could hit health - and came up with the model used by Centre for Cities to calculate mortality rates linked to air quality.
Public Health Wales has said air pollution contributes to 1,400 deaths a year.
Overall, the health body said that air quality impacts on health contributes to 6% of deaths across Wales.
Researchers at Kings College London also set about examining how air quality could affect health outcomes.
Their research published last year found:
- Living near a busy road in London may contribute to 230 hospital admissions for strokes every year
- Living near a busy road may stunt lung growth in children by 12.5% in London and 14.1 % in Oxford
- In Birmingham, the risk of outside-hospital cardiac arrest is 2.3% higher on high pollution days
- Higher air pollution days are responsible for 43 more people going to hospital for respiratory disease in Southampton, 68 in Bristol, 98 in Liverpool.
The Centre for Cities report called on the Welsh Government to act to combat the pollution.
It wants Wales to adopt stricter World Health Organisation guidelines on levels of PM2.5 pollution. According to its report, 62% of roads monitored in UK cities exceed the annual recommendation for PM2.5, of 10 micrograms per cubic metre.
The current target adopted by Wales is 25 micrograms per cubic metre.
The think tank has also called for a ban on using wood burning stoves and coal fires in areas where air pollution exceeds guidelines.
It has also said ministers should introduce ultra-low emission zones in city centres - and charge car and van drivers to enter them.
Cardiff council announced last week that it was already examining proposals for congestion charge across the city.
Responding to the report, the Welsh Government said it was working on "ambitious actions" to reduce air pollution.
Official said the Clean Air Plan for Wales includes measures to reduce emissions from domestic burning, consulting on new targets for particulate matter in Wales, and reducing cross border pollution.
It said £21m was also being used to help Cardiff council combat nitrogen dioxide levels to comply with EU limits on pollution.