High levels of ozone may be killing off people who live in cities around the world, experts suspect.
Heavy traffic exhaust contributes to the problem
Peaks in air ozone levels were linked with peaks in premature death rates in urban areas across the US, researchers at Yale University found.
Ozone pollution, from vehicles and power stations, has been linked before to increased hospital admissions and lung diseases such as asthma.
The findings appear in the Journal of the American Medical Association.
The researchers compared death rates with ozone levels for 95 large urban US communities between 1987 and 2000.
They found an increase of 10 ppb (parts per billion) in the previous week's ozone was associated with a 0.52% increase in the overall daily death rate and a 0.64% increase in cardiovascular and respiratory-related deaths.
This would translate to 319 extra deaths in New York City and nearly 4,000 extra deaths across the 95 areas studied in the year 2000 alone, the researchers calculate.
Lead researcher Dr Michelle Bell said: "This actually underestimates the total impact of ozone on mortality because it only captures the mortality impact associated with high ozone levels in the past few days, not the impact associated with a lifetime exposure."
The average daily level in the study was 26 ppb.
The Department for Environment Food and Rural Affairs (DEFRA) in the UK has said ozone levels should not exceed more than 100ug/m3 (51 ppb) more than 10 times per year by the end of December 2005.
A spokesman from the Met Office said: "In the UK, we already have similar statistics on ozone levels increasing and the health response.
"It is still not clear if there is a threshold concentration, below which there is no health impact.
"Some researchers believe there is no safe level."
"Ozone is not emitted directly into the atmosphere. It is a secondary pollutant produced by reactions between different chemicals and sunlight in the atmosphere.
"We see certain times of the year when we have higher levels of ozone and it's often when we get heat wave conditions."
He said DEFRA published forecasted levels of ozone, so people particularly susceptible to ozone levels - people with lung conditions such as asthma - could follow these and take extra medication to cover themselves for those high periods.
He said cutting emissions from vehicles and industrial activities was one way to reduce ozone pollution, although there are also natural sources of ozone precursors.
Professor Andrew Peacock of the British Thoracic Society said: "We know that in the US high ozone levels cause lung health problems, but we are also concerned about levels of ozone here in the UK, particularly in cities where there is a lot of exhaust pollution.
"When ozone is inhaled at ground level it can be harmful to the lungs and cause chronic respiratory diseases such as COPD and asthma.
"Adequate funding must be given to research into this area so that we can fully understand the effect of ozone on the health of people in the UK."