(NBC NEWS) - A new government study reveals air pollution may increase the risk of pre-term birth for women with asthma.
Researchers found long and short-term exposure to nitrogen oxide and carbon monoxide were especially risky just before conception and in early pregnancy.
The last six weeks of pregnancy is another vulnerable window for women with asthma.
Exposure to pollution from acids, metals, and dust in the air during this time was also linked to a higher risk of pre-term birth.
Experts said expectant mothers with asthma may want to limit their outdoor activities when the forecast calls for poor air quality.
This study was led by researchers at the National Institutes of Health and published in 'Journal of Allergy and Clinical Immunology.'