A new study finds one-third of young adults are vulnerable to severe COVID-19. Smoking may help explain the risk, CNN reports.

In otherwise healthy 18- to 25-year-olds, smoking is a bigger risk factor for severe COVID-19 than obesity or asthma, the researchers report in the Journal of Adolescent Health.

The study included a nationally representative sample of 8,400 young adults. The researchers analyzed their medical vulnerability to the virus in relation to risk factors set out by the Centers for Disease Control and Prevention. The study found 32% of the study population was medically vulnerable for severe COVID-19. When study participants who smoked cigarettes or e-cigarettes were removed from the analysis, the medically vulnerable population dropped in half, to 16%.

“Recent evidence indicates that smoking is associated with a higher likelihood of COVID-19 progression, including increased illness severity, ICU admission or death,” lead researcher Sally Adams said in a news release. “Smoking may have significant effects in young adults, who typically have low rates for most chronic diseases.”