California's History with Smog

California has more registered vehicles than any other state, and for most of the last century it has had some of the dirtiest air in the country to match. The Smog Check program exists because of that combination. This is the short version of how the state got here and why your car gets tested every other year.

What smog is

Smog is mostly ground-level ozone. It forms when nitrogen oxides (NOx) and hydrocarbons (unburned fuel and other volatile organic compounds) react in sunlight. Car exhaust is a major source of both, and Southern California's sunshine and the mountains that trap air over the Los Angeles basin make the reaction especially efficient there.

What a smog check actually measures

It depends on the age of the vehicle.

• 1999 and older gasoline vehicles get a tailpipe test. The analyzer reads five gases: hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), carbon dioxide (CO2) and oxygen (O2). HC, CO and NOx are the pollutants being graded; CO2 and O2 help the analyzer confirm it is reading a valid exhaust sample.
• 2000 and newer gasoline vehicles, 1998 and newer diesels, and hybrids are tested through the onboard diagnostics (OBD) computer instead of the tailpipe. Since March 9, 2015, California has required its OBD-focused inspection system for those vehicles. Hybrids began needing smog checks in April 2015.

Every inspection also includes a visual check of the emission control parts and, on most vehicles, a functional check of things like the gas cap and the check engine light. If you want to see where your own car stands, how to pass a California smog check walks through it.

The health cost

Diesel exhaust was the first big target on the health side. In 1998 the California Air Resources Board (CARB) identified diesel particulate matter as a toxic air contaminant. Later research tied fine-particle pollution to respiratory illness, heart disease, asthma attacks and cancer.

The bills are measurable. A 2010 RAND Corporation study led by economist John Romley estimated that failing to meet federal air quality standards caused nearly 30,000 hospital admissions and emergency room visits in California from 2005 to 2007, at a cost of about $193 million to public and private insurers. The most common visit was an emergency room trip for a child's asthma attack. Los Angeles and Orange counties accounted for more than half of the total.

Children are a particular concern. The University of Southern California's Children's Health Study, which followed thousands of Southern California schoolchildren, found that kids who played several team sports in high-ozone communities were more likely to develop asthma than kids in the same communities who did not.

California still leads the country in people living with air that fails federal standards. The American Lung Association's annual State of the Air report regularly puts Los Angeles, Bakersfield, Visalia and Fresno among the most polluted metro areas in the United States for ozone and particle pollution.

The climate side

Smog and greenhouse gases are different problems, but cars feed both. Carbon dioxide does not cause smog; it is a greenhouse gas, and transportation is the largest source of greenhouse gas emissions in California's statewide inventory. That is why the state now regulates vehicles on both fronts: smog-forming pollutants through the Smog Check program and emission standards, and greenhouse gases through its own vehicle standards and the push toward zero-emission cars.

The state's own climate assessments expect more frequent extreme heat, rising sea levels along the coast, and pressure on water supplies and agriculture. Hotter days also make ozone form faster, so a warmer climate makes the smog problem harder to solve.

Why it comes down to your car

Newer cars are dramatically cleaner than older ones, but a single car with a failing catalytic converter or a disconnected sensor can emit many times what it was built to. The Smog Check program exists to find those cars and get them repaired. If yours fails, here is what to do next, including the state's Consumer Assistance Program for repair help.

Originally published on SmogCheckDeals.com in June 2018; updated in October 2026 to correct outdated figures and testing rules.