"If you are observed smoking and are cited, it's a $100 fine," said state Senator Jenny Oropeza.
State Senator Oropeza authored the "Smoke-free Cars with Minors" law.
"I think that's a great law, because it protects our kids who are basically held hostage in a vehicle," said Oropeza. "They don't have a choice to get out of that car."
To prove this point, health experts used a volunteer smoker and sophisticated sensors to measure just how toxic the air can get when you're in a car with a smoker.
"It went way above the EPA hazardous limit," said Stanford environmental scientist Dr. Neil Kelpeis. "Over 6,000 micrograms per unit cube, which is about 30 times the hazardous level set by the EPA."
Even when you cracked the windows open, the levels are still considered hazardous.
"They were 10 to 20 times that EPA limit, so a child in the backseat would still be exposed to this very high level of particulate matter," said Dr. Kelpeis.
Pediatricians say kids are more susceptible to the harmful effects of secondhand smoke because their bodies are still developing. Some of the problems include asthma, bronchitis and even ear infections.
"Pound for pound, they breathe more air in a minute-by-minute basis. So that in any given environment, children are actually sucking in more air pollution than adults in the very same environment," said Dr. Mark Horton from the California Department of Public Health.
Studies show secondhand smoke contains 50 cancer causing chemicals, and long-term exposure is linked to heart disease and lung cancer in adults.
"Secondhand smoke kills. Don't smoke in a car with a kid," said Oropeza. "Today it's against the law."