A new study finds parents greatly underestimate their children’s exposure to secondhand smoke. While 13 percent of parents said their children were exposed to cigarette smoke, blood tests showed the rate was 55 percent.

In children, secondhand smoke can cause ear infections, more frequent and severe asthma attacks, respiratory symptoms and infections, and a greater risk of Sudden Infant Death Syndrome, according to the Centers for Disease Control and Prevention.

The researchers at the University of California, San Francisco, tested 496 blood samples from young children. The study tested blood levels of cotinine, a chemical produced by the body after it is exposed to nicotine, Reuters reports.

In the Archives of Pediatrics and Adolescent Medicine, the researchers noted children do not necessarily need to be around someone who is smoking in order to be exposed to secondhand smoke. They can be exposed if they spend time in a room where someone has recently been smoking.

Cotinine testing is not readily available to the general public, according to study co-author Dr. Neal Benowitz.