History Repeats Itself: Revisiting PM2.5 Air Pollution in Beijing, China, and the London Smog

In December 2015, a serious situation unfolded in Beijing, China, as atmospheric PM2.5 concentrations surpassed several hundred μg/m ³ , prompting the first-ever red alert restricting outdoor activities. This severe air pollution was broadcast by media worldwide with striking images (see Photo 1), leaving a vivid impression on people globally. However, it may be surprising that few people are aware that a similar severe air pollution event occurred more than 60 years ago.

A prime example of historical air pollution in a major city is the London Smog (see Photo 2), which occurred in London, UK, 65 years ago in December 1952. It was a catastrophic event estimated to have caused over 10,000 deaths. It is astonishing that an event not caused by natural disasters like earthquakes, tsunamis, or volcanic eruptions, nor by epidemics or war, could result in such a large number of fatalities.

The severe air pollution in Beijing and London shares many common factors. In both cases, record-high levels of air pollution were caused by the massive release of pollutants like sulfur dioxide (SO ₂ ) and dust from coal heating in winter, combined with atmospheric stagnation due to a temperature inversion. In high-latitude cities like Beijing and London, surface temperatures drop below freezing in winter. The air near the ground cools, becomes denser, and stagnates over the surface, inhibiting the dispersion of air pollutants and making high-concentration pollution events more likely.

Furthermore, a key point about the London Smog is that it was not merely the high concentration of dust; large amounts of sulfur dioxide also formed highly acidic particles (Acid Particles), which caused acute bronchitis and pneumonia in those who inhaled them. At the time, there were no government restrictions, such as bans on going outdoors, and the elderly, children, and the sick accounted for many of the victims. Today, the health concerns of PM2.5 air pollution primarily focus on soot from diesel exhaust, carcinogens, and toxic metals within the particles, with the health effects of these chemical components linked to long-term exposure. On the other hand, the danger of the highly acidic particles (Acid Particles) that caused acute illnesses during the London Smog is not widely considered. While high concentrations of PM2.5 are a major issue in Beijing, the acidity (pH) of this PM2.5 has not been a primary concern. In China, progress on reducing sulfur dioxide (SO ₂ ) and dust emissions from coal-based heating and thermal power plants has been limited. Consequently, it is highly probable that, similar to the London Smog, large quantities of highly acidic particles (Acid Particles) capable of causing acute illnesses are currently being generated. Evidence for this comes from chemical analyses of PM2.5, which have measured extremely high concentrations of sulfates. Therefore, the most immediate problem in cities like Beijing is the presence of these highly acidic particles (Acid Particles) that cause acute illnesses, making it essential to measure the acidity (pH) of PM2.5.

Additionally, during the London Smog 65 years ago, modern measuring instruments were not available, and data on air pollutants was limited. By measuring the acidity (pH) of PM2.5 during high-concentration pollution events in Chinese megacities like Beijing and confirming the presence of highly acidic particles (Acid Particles), we can re-examine the actual conditions of the London Smog from 65 years ago.