Curbing outdoor air pollution could help prevent 2.7 million premature births annually - a condition that threatens children's lives and increases their risk of long-term physical and neurological problems, scientists said on Thursday.

Fine particles in the air from diesel fumes, fires and other sources, might increase the risk of premature births - alongside other risks including a mother's age and health - according to a study published in the journal Environment International.

"Air pollution may not just harm people who are breathing the air directly - it may also seriously affect a baby in its mother's womb," said Chris Malley, lead author of the study which is based on data for 2010.