Some cot deaths could be linked to babies being exposed to pollutants, researchers have suggested.
Putting a baby to sleep on its back reduces cot death risk
A US study estimated 16% of cot deaths could be attributed to PM10s - fine sooty particles found in exhaust fumes.
However, the study in Environmental Health: A Global Science Source did not say how pollution may cause cot deaths.
British experts said if pollution was a cause, it would account for a tiny proportion of cot deaths compared to parental smoking and sleeping position.
The researchers looked at death rates in infants aged one to 12 months amongst 700,000 babies born across the US between 1995 and 1997.
They also examined data on PM10 levels from the US Environmental Protection Agency. The chemical was used because it provides a marker of general pollution levels.
There were 236 deaths per 100,000 infants from all causes.
Of these, 14.7 per 100,000 could be attributed to an average exposure to PM10 pollution.
In the case of unexplained infant deaths, the figure was 11.7 per 100,000 - 16% of the total.
The researchers also estimated that 6% of deaths from all causes and 24% of deaths from respiratory diseases could be attributed to PM10s.
They add that three quarters of these deaths were in areas where PM10 levels were within recommended levels - which are roughly the same as in the UK.
The team, led by Dr Reinhard Kaiser of the University of Basel in Switzerland, accept their study is based on assumptions, rather than hard data.
But writing in Environmental Health: A Global Health Access Resource, they say: "Evidence for a causal effect of air pollution on morbidity and mortality is strong for adults, and evidence is building that air pollution has an effect on infants and young children and a potential impact during the foetal period.
How to cut the risk:
Cut out smoking during pregnancy
Fathers should stop smoking at this time too
Do not let anyone smoke in the same room as your baby
Place your baby on its back to sleep
Do not let your baby get too hot
Keep your baby's head uncovered
Place your baby with its feet to the foot
of the cot
Preferably sleep your baby in a cot in your bedroom for the first six
Do not sleep with a baby on a sofa, armchair or settee
If your baby is unwell, seek medical advice promptly
"The evidence needs to be constantly reviewed as further studies become
"Our estimates are based on the best currently available information,
leaving considerable uncertainty about the size of the true effect of
particulate matter on infant mortality.
"However, given that the whole population is exposed, we conclude that air
pollution-related infant mortality is a major public health problem."
Cot death affects about 300 infants a year in Britain. Known risk factors include the baby's sleeping position, temperature, and exposure to cigarette smoke.
'Reduction is desirable'
Professor George Haycock, consultant paediatrician and FSID's scientific advisor, said: "The US study has found a clear association between deaths in infancy from all causes and the levels of PM10 particles.
"What was depressing was that three quarters of the excess deaths occurred in places where PM10 levels are within what's deemed to be acceptable."
But he said the study did not show if it was PM10s or other components of pollution which could increase cot death risk.
Professor Haycock added: "If there is a risk from pollution, it is tiny in comparison with the risk of parental smoking and with putting your baby to sleep on its front."
He added: "It would be difficult to argue against the authors' inference that a reduction in the current recommended acceptable levels of air pollutants is highly desirable.
"The authors were cautious enough not to extrapolate their conclusions to countries other than the USA but it seems highly unlikely that this phenomenon is unique to North America."