Air pollution during pregnancy linked to low birth weight Prof Tanja Pless-Mulloli, who led the UK arm of the study, said: "These microscopic particles, five times smaller than the width of a human hair, are part of the air we breathe every day. What we have shown definitively is that these levels are already having an effect on pregnant mothers. While much has been done to improve air quality, this study shows we can't be complacent as we've shown that clean air is really important for the health of our newborns. The effect on health is most likely a result of the size of the particles, which can penetrate our airways and are not easily expelled like other foreign bodies, and of various chemicals found on their surface, she added. Dr Tony Fletcher, senior lecturer in Environmental Epidemiology at the London School of Hygiene and Tropical Medicine, who was not involved in the study, said: "This new study is very helpful in establishing another health impact of air pollution. The study is of excellent quality and the conclusions are clear: while the average effect on each baby is small and so should not alarm individual prospective parents, for the whole population these small risks add up across millions of people.