Exposure to secondhand smoke increases the risk of wheezing and asthma in children and teens by at least 20 percent, according to a review of previous studies. The researchers of the new analysis say preventing parental smoking is crucially important to the prevention of asthma.

The researchers found the biggest effect of secondhand smoke exposure on asthma risk was in babies and toddlers whose mothers smoked during pregnancy or soon after the children were born, Reuters reports.

The analysis, published in Pediatrics, included more than 70 studies published between 1997 and 2011. All of these studies followed children as they grew, to determine which ones were diagnosed with wheezing or asthma.

According to the Environmental Protection Agency, children’s developing bodies make them more susceptible to the effects of secondhand smoke. Due to their small size, children breathe more rapidly than adults, and breathe in more secondhand smoke. Children who breathe in high doses of secondhand smoke, such as those with parents who smoke, run the greatest risk of experiencing harmful health effects.