History: from London smog to the Six Cities Study
In December 1952 a mix of coal smoke and fog, the Great Smog, blanketed London for five days, killing about 4,000 people outright and more than 10,000 over the following months. It led to Britain's Clean Air Act of 1956. But the decisive evidence that invisible small particles were the real killer came in 1993 from Douglas Dockery's Harvard team. Their Six Cities Study found residents of the dirtiest city had 26% higher mortality than those in the cleanest, and PM2.5 explained the gap best. The US introduced a PM2.5 standard in 1997, and in 2013 the International Agency for Research on Cancer classified outdoor air pollution and particulate matter as Group 1 carcinogens.
Health effects and limits
PM10 lodges in the nose and airways, while PM2.5, about one-thirtieth the width of a hair, reaches the alveoli and partly enters the bloodstream. That links it not only to worse asthma and allergic rhinitis but also to lung cancer, heart attacks and stroke. In 2021 the WHO halved its annual PM2.5 guideline from 10 to 5 µg/m³. Much of Korea's fine dust is "secondary" particles formed in the air from the same sulfur and nitrogen oxides behind acid rain, rather than soot emitted directly.
| Korean PM2.5 band | 24-hour level (µg/m³) | Advice |
|---|---|---|
| Good | 0–15 | Normal activity |
| Moderate | 16–35 | Sensitive groups limit long outdoor exertion |
| Bad | 36–75 | Cut outdoor time; wear a KF80+ mask |
| Very bad | 76+ | Stay indoors; close windows; run a purifier |
Everyday protection
On bad days a certified KF80 or KF94 mask (similar to N95/FFP2) sealed around the nose and chin works well; surgical and cloth masks filter little PM2.5. Indoors, ventilate briefly and then run an air purifier, and always use the range hood when cooking, since grilling or frying can push indoor PM2.5 above outdoor "very bad" levels. Washing your hands and face after coming home and drinking enough water also help. Ground-level ozone alerts, unlike fine dust, peak on summer afternoons and can't be filtered by a mask.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- Should I open the windows on high-dust days?
- Keeping them shut all day lets CO₂ and cooking fumes build up. On "bad" days, ventilate about three times a day for roughly ten minutes and run an air purifier.
- Does all of Korea's fine dust come from China?
- It varies by season and day. During winter and spring peaks foreign sources can account for around half, but on an annual basis domestic power, traffic, industry and secondary formation also matter a lot.
Sources reviewed: 2026-10-26
References
- Dockery, D. W., et al. (1993). An association between air pollution and mortality in six U.S. cities. New England Journal of Medicine, 329(24), 1753–1759.
- World Health Organization (2021). WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide.
- IARC (2013). Outdoor air pollution a leading environmental cause of cancer deaths. Press release No. 221.