By Naomi Thomas, CNN

(CNN) -- One in three young adults is at risk of severe Covid-19, and smoking plays a big part in that risk, according to new research published Monday in the Journal of Adolescent Health.

Researchers at the University of California, San Francisco, looked at more than 8,000 participants ages 18 to 25 who had participated in the National Health Interview Survey to see what their medical vulnerability to severe Covid-19 was in relation to risk indicators that had been set out by the US Centers for Disease Control and Prevention, including health conditions and smoking habits.

The researchers found 32% of the total study population were medically vulnerable for severe Covid-19. However, when the group of participants who smoked cigarettes or e-cigarettes were taken out of the analysis, the medically vulnerable percentage decreased by half, to 16%.

"The difference between estimates is driven largely by the sizeable portion of young adults who reported that they engaged in past 30-day smoking (1 in 10) and past 30-day e-cigarette use (1 in 14)," the report said. "By contrast, relatively fewer young adults reported medical conditions identified by the CDC as conferring severe illness risk."

The research showed that in the whole study population, young adult men were at a higher risk for severe Covid-19. Although more women reported having asthma and immune conditions, higher rates of smoking in men overrode this. However, looking at just the nonsmokers, women had a higher risk.

"Recent evidence indicates that smoking is associated with a higher likelihood of COVID-19 progression, including increased illness severity, ICU admission or death," said Sally Adams, lead author of the study and a specialist at University of California, San Francisco's National Adolescent and Young Adult Health Information Center, in a press release. "Smoking may have significant effects in young adults, who typically have low rates for most chronic diseases."

Another interesting finding from the research is that in the 18-to-25 age group, White young adults had the highest vulnerability.

"Our finding of lower medical vulnerability of racial/ethnic minorities compared with the white subgroup, despite controlling for income and insurance status, was unexpected," the study said. "It is also inconsistent with research showing higher rates of Covid-19 morbidity and mortality and other chronic illnesses among racial/ethnic minorities, specific to one age group."

It's also inconsistent with the 15-to-24 age group, the researchers said, in which Hispanic and Black Americans were shown to have the highest rates of Covid-19 deaths.

"This suggests that factors other than the CDC's medical vulnerability criteria play a role in the risk of severe Covid-19 illness in the young adult population," the researchers said in the study.

The study does have some limitations, including the lack of information about Covid-19 in the 18-to-25 population, and a chance that it could underestimate the vulnerability rates for certain ethnic or racial subgroups of young adults due to the data source.