California's Smog History, From 1943 to Smog Check

California's smog history starts in 1943, when the first recognized smog episodes hit Los Angeles and visibility fell to three blocks. By the early 1950s, car exhaust was identified as the cause. That led to tailpipe standards in 1966, the Air Resources Board in 1967 and, as of October 2026, nearly a million smog checks a month.

This is the short version, with dates checked against the California Air Resources Board (CARB) and the Bureau of Automotive Repair (BAR).

When did smog start in California?

Smog was first recognized in California in Los Angeles in the summer of 1943. CARB's history describes visibility of only three blocks, with people suffering burning eyes, burning lungs and nausea. The city called it a "gas attack" and blamed a nearby butadiene plant, but when that plant shut down, the smog stayed.

In 1947 Los Angeles County formed the first air pollution control district in the nation. It went after the obvious sources, smoke-belching power plants and oil refineries, and the smog still persisted. Something else was feeding it.

What causes smog, and who figured it out?

Smog in California is mostly ground-level ozone, formed when hydrocarbons and nitrogen oxides from engines react in sunlight. Dr. Arie Haagen-Smit, a Caltech chemistry professor, worked this out in the early 1950s in a Los Angeles air district lab, and named automobile exhaust as the main culprit. His work became the foundation of modern air pollution rules.

Two ingredients in car exhaust were to blame: hydrocarbons from gasoline and the nitrogen oxides (NOx) that combustion engines produce. Southern California supplied the rest, with steady sunshine and mountains that hold air over the Los Angeles basin. Haagen-Smit later became the first chair of CARB, serving from 1968 to 1973.

How did California start regulating car emissions?

California began regulating car emissions in 1966, when it set the nation's first tailpipe standards for hydrocarbons and carbon monoxide. On August 30, 1967, Governor Ronald Reagan signed the Mulford-Carrell Air Resources Act, creating the California Air Resources Board from a merger of the Bureau of Air Sanitation and the Motor Vehicle Pollution Control Board.

The same year, the federal Air Quality Act of 1967 let California set stricter rules than the rest of the country because of its geography, weather and growing number of cars. The federal Clean Air Act of 1970 kept that authority in place, and California used it again and again.

CARB also eliminated lead in gasoline and adopted standards for cleaner-burning gasoline and diesel fuel. By its account, California cars became the cleanest in the world in the 1980s and 1990s.

How has the Smog Check program changed over time?

The California Smog Check program has been rebuilt several times as cars changed. Assembly Bill 2018 (1994) required loaded-mode tailpipe testing in the most polluted areas. Assembly Bill 2289 (2010) moved 2000 and newer cars to an on-board computer inspection and created the STAR program, which began January 1, 2013. Hybrids joined in January 2015.

• 1994, AB 2018. Passed in response to amendments to the federal Clean Air Act, it required a loaded-mode test on a dynamometer in "Enhanced" areas that miss federal air quality standards. That test can measure nitrogen oxides, which a simple idle test cannot.
• 2010, AB 2289. Switched most 2000 and newer vehicles to an OBD-focused inspection and created the STAR certification for stations that meet higher performance standards. STAR replaced the earlier Gold Shield program.
• 2015. Hybrids became subject to Smog Check, according to BAR's reference guide.
• 2019. New cars got eight model years before their first renewal test, up from six, paying a smog abatement fee instead.
• 2025. A BAR rule effective October 1, 2025 requires all readiness monitors that can feasibly be set to be complete, phased in by vehicle.
• 2028. SB 1392 (Chapter 245, Statutes of 2026) will exempt qualifying collector cars built before the 1981 model year, rolling forward to pre-1986 models by 2033.

Today gasoline, hybrid and alternative-fuel cars from 1976 and newer are covered, along with diesels from 1998 and newer. Our smog check requirements guide has the current rules.

What does a smog check measure today?

A California smog check today is mostly a computer check. For most 2000 and newer gas cars and 1998 and newer diesels, the station reads the on-board diagnostic system. For 1999 and older gas cars, a tailpipe probe measures hydrocarbons, carbon monoxide and, in most Enhanced areas, nitrogen oxides. Every test adds visual and functional checks.

The numbers from BAR's August 2026 Executive Summary show the split:

Older cars fail more than twice as often. A check engine light that is on is an automatic fail. Our guide on how to pass a California smog check covers what trips cars up, and what happens during a smog check walks through the test.

Is California's air still a problem?

California's air is far cleaner than in 1943 but still not clean enough. CARB says air quality keeps improving even as population and car numbers grow, yet California "still lags behind the rest of the nation," as growth, reliance on driving and sunshine keep feeding smog. Areas that miss ozone standards get BAR's strictest Smog Check rules.

The health stakes are well documented:

• Diesel exhaust. In 1998 CARB identified diesel particulate matter as a toxic air contaminant, based on evidence linking it to lung cancer and other health effects.
• Children. 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.

How are smog and climate change connected?

Smog and climate change are separate problems with a shared source: engines. Carbon dioxide does not cause smog, but it is the main greenhouse gas, and cars emit it alongside the hydrocarbons and NOx that do form smog. In 2006, Assembly Bill 32, the Global Warming Solutions Act, gave CARB responsibility for cutting California's greenhouse gas emissions too.

That is why California now regulates cars on both fronts: smog-forming pollution through emission standards and Smog Check, and greenhouse gases through vehicle standards and its zero-emission vehicle rules.

Why does it come down to your car?

New cars are dramatically cleaner than the ones that filled the Los Angeles sky in the 1940s, but a single car with a failed catalytic converter or a disconnected sensor can pollute far more than it was built to. The Smog Check program exists to find those cars and get them repaired before they add to the problem.

If your car fails, here is what to do next, including BAR's Consumer Assistance Program for repair help. When your test is due, find a smog check near you.

Sources: CARB: History · CARB: Overview of diesel exhaust and health · BAR: Smog Check · BAR: STAR program FAQ · BAR Smog Check Executive Summary, August 2026 · BAR: OBD readiness monitor regulations · SB 1392 (2026)

Figures checked against BAR, DMV and CARB sources as of October 2026.