Secondhand smoke hurts everyone who breathes. When those who breathe are kids, and when where they're breathing is the family car, it's cause for special concern.

A 2006 study by researchers at the Harvard School of Public Health found “alarming” levels of secondhand smoke were generated in just five minutes in vehicles under different driving, ventilation, and smoking conditions. The researchers concluded that “smoking in cars under typical driver and traffic conditions provides potentially unsafe secondhand smoke exposure.” 
In fact, the average levels of pollution inhaled from secondhand smoke in the vehicles was higher than levels that were found in similar studies of smoking in bars in several towns in eastern Massachusetts. The levels of particulate matter found in the vehicles were greater than levels that the U.S. Environmental Protection Agency has described as “unhealthy for sensitive groups” such as children and the elderly. 

The pollution levels detected “highlight the potentially serious threat to children’s health presented by secondhand smoke in private cars under normal driving conditions,” according to the report. 

The researchers also found a “significant increase” in levels of carbon monoxide. Carbon monoxide “is a poisonous gas, which may cause coma and death in large amounts, but among infants is known to induce lethargy and loss of alertness even in small quantities,” according to the report. 

Tobacco Free Nebraska has highlighted the problem of secondhand smoke in cars in this video.