Photochemical Smog Day

What Is Photochemical Smog Day?

Every July 18th is "Photochemical Smog Day" (光化学スモッグの日, Kōkagaku Sumoggu no Hi) in Japan. On July 18, 1970, Japan's first documented health damage from photochemical smog occurred in Suginami Ward, Tokyo, when 43 students at Tokyo Risshō Junior and Senior High School, near the Kan-nana (Ring Road 7) highway, complained of eye irritation, sore throats, and other symptoms during a PE class on the school grounds. This incident — which shocked the public and prompted urgent government action — is remembered as a milestone in Japan's environmental awakening.

What Is Photochemical Smog?

Photochemical smog forms when sunlight triggers chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) — both emitted by vehicle exhaust and industrial processes — producing ozone and other harmful oxidants. In high concentrations, these oxidants cause eye and throat irritation, respiratory distress, and at severe levels can be life-threatening for people with respiratory or cardiovascular conditions. Photochemical smog is worst on hot, sunny, windless days — conditions common in Tokyo's summer.

Japan's Environmental Crisis of the 1960s–70s

The 1970 Suginami incident came at the peak of Japan's "Kōgai" (公害, pollution damage) crisis — a period when Japan's rapid industrialization had produced catastrophic environmental and public health consequences. Minamata disease (mercury poisoning from industrial wastewater), Itai-itai disease (cadmium poisoning), and Yokkaichi asthma (sulfur dioxide from petrochemical plants) had already galvanized environmental activism. The 1970 photochemical smog incident added air pollution in major cities to the list of urgent environmental emergencies demanding government response.

Japan's Environmental Response

The Japanese government responded to the environmental crisis with landmark legislation. The Basic Law for Environmental Pollution Control (1967) was strengthened, the Environment Agency was established in 1971, and strict emissions standards for vehicles and industrial facilities were introduced. Japan's automobile exhaust regulations — the "Muskie Law" equivalent — were among the world's toughest. The result was a dramatic improvement in urban air quality over the following decades, with the nationwide number of photochemical smog alert days (counted by prefecture) falling from a peak of 328 in 1973 to around 50–100 per year in recent years.

Fun Facts

• Photochemical smog alerts: Tokyo's metropolitan government still monitors ozone levels and issues "photochemical smog alerts" when concentrations exceed safe thresholds — though such alerts are now rare compared to the 1970s peak.
• Where it started: Photochemical smog was first identified in Los Angeles in the 1940s.
• Global lesson: Japan's experience of industrial pollution crisis and subsequent environmental cleanup became an international model — demonstrating that environmental protection and economic growth are not incompatible.