PM2.5 refers to tiny particles in the air that are 2.5 micrometers or less , about thirty times smaller the width of a human hair. Because they are so , they can travel deep the lungs and even enter the . In northern and central Thailand, levels rise during the dry , when crop burning increases and still air pollution near the . Long-term exposure is linked to heart and lung .

In December 1952, a thick, yellow fog settled over London for five days. Cold, still air trapped smoke from millions of coal fires and factories close to the ground. Visibility fell so low that buses stopped running and some people could not see their own feet.

At first, few people realized how deadly the event was. Only later did officials count the deaths: at least 4,000 people died during the smog and in the days that followed, and later estimates suggest that the total may have been far higher. Most of the victims were elderly people or those with breathing problems.

The disaster led to the Clean Air Act of 1956. The law created zones in which only smokeless fuels could be burned, offered money to help households convert their fireplaces, and encouraged moving power stations away from cities. Over the following decades, smoke pollution in London fell dramatically.

London’s experience is often cited in discussions of air pollution in Asian cities today. The sources of pollution are different, including vehicles and agricultural burning, but the lesson is similar: dangerous air is not simply a fact of nature, and strong public policy can improve it.