China’s Air Pollution and Transboundary Environmental Impacts on the Health of People in Japan

Pollution abroad is not someone else’s problem

Respiratory diseases are increasing among people in Japan, particularly over the past two years. More precisely, these cases are becoming more noticeable. In 2024, infectious diseases were circulating year-round. While that trend eased in 2025, respiratory illnesses—especially in summer—have become strikingly severe.

In Kawasaki and Yokohama, photochemical smog used to be a summer fixture during Japan’s high-growth era. It receded after environmental controls, but in recent years Kawasaki has again issued occasional photochemical smog alerts. According to people working around Haneda Airport, Kawasaki’s alerts have been almost daily in the summer of 2025. It’s not just Kawasaki: residents of Shinagawa and Ōta wards will have heard multiple loudspeaker announcements about photochemical smog this summer.

We are told that South Korea is facing even worse conditions than Japan, and China worse still. Asian dust driven by desertification in China and overall environmental/air pollution ride the prevailing westerlies and affect countries beyond China, so this is not someone else’s problem for Japan.

During COVID—especially under China’s “zero-COVID” policy—non-COVID respiratory infections were relatively few thanks to strict public-health measures, even if SARS-CoV-2 itself was circulating. Once China lifted zero-COVID and restarted factories, downstream effects began appearing in Japan.

With China’s economic growth over recent decades, hay fever has become more common in Japan and air purifiers have spread widely, yet many still thought of China’s pollution as a distant issue. The post-COVID reopening changed that. Looking back, that period felt “clean”: in medicine, “clean” and “unclean” have specific meanings tied to pathogenic risk, which differ from everyday usage.

2024: A year packed with infections

In 2024, some infectious disease seemed to be circulating in every season. Because semi-isolated lifestyles reduced contact with a range of pathogens, we considered that immune “boosting” via exposure might have waned. At the same time, the tourism surge brought infections from all over the world—including the southern hemisphere, where seasons are reversed—possibly fueling off-season outbreaks.

Historically in Japan, influenza strains were often traced to China where birds, pigs, and humans coexist; vaccines were developed in anticipation of “Hong Kong type” strains entering Japan. Now influenza sometimes spreads even in summer. With current labels like “Victoria lineage,” strains may also be arising from livestock regions such as Australia. In a globalized world, diseases that emerge in one place can spread worldwide. That was my working view—until 2025 began to look a bit different.

Shifts seen in 2025

In 2025, unlike 2024, we did not see a constant parade of infections. Summer colds remain common and COVID is circulating, but spring and early summer were calmer than 2024.

I wondered if we were returning to pre-COVID patterns. Yet, as in previous years, we saw conspicuous cases from around May–July: chronic cough of unclear cause, upper-airway pain/irritation, and worsening respiratory disease—even after cedar and cypress pollen season had ended. Summer colds were not rare.

Pertussis-like illnesses often surge this time of year, and some cases likely were that, but many patients had lingering, undiagnosed cough. Asthma also increased. I suspect connections between childhood asthma and panic disorder; we saw a fair number of such cases. Overall, thanks perhaps to work-style reforms and remote work, both onset and course of panic disorder have seemed to improve in recent years. In central Tokyo, triggers often involve trains—enough to call it “train disease.” Strictly speaking, when a clear trigger exists, the diagnosis shifts from panic disorder to agoraphobia or a specific phobia.

Reports suggest even stronger impacts in China and nearby South Korea, implying these patterns relate to China’s lifting of zero-COVID and the restart of industrial production.

Pollutants transported from China

Over the long term, hay fever has been rising in Japan, and owning an air purifier has become normal. Below is a broad list of air and environmental pollutants transported from China:

PM2.5 (fine particulate matter): particles ≤2.5 μm in diameter.
Asian dust (Kosa)–related substances: desertification is increasing emissions. This dust mixes with other aerosols and may carry heavy metals; it also adsorbs sulfur oxides and nitrogen oxides.
Heavy metals: lead, cadmium, arsenic. Lead poisoning is well known; historical cases are noted among Chinese emperors, in Roman times, and with Newton. Cadmium is infamous from “Itai-itai disease.” Arsenic is toxic but is sometimes used as a drug for certain hematologic malignancies.
Gaseous air pollutants / photochemical oxidants: sulfur oxides (SOx, e.g., SO₂), nitrogen oxides (NOx), volatile organic compounds (VOCs), ozone (O₃). These are implicated in photochemical smog and acid rain. Photochemical reactions in air generate ozone and secondary particles (sulfates, nitrates), potent photochemical oxidants that drive airway hyper-responsiveness. Transboundary ozone is also an issue.
Ammonium salts: often grouped among harmful organic compounds.
Hazardous chemicals / polycyclic aromatic hydrocarbons (PAHs): carcinogenic.
Mercury: described here as petroleum-derived. Organic mercury is known from the Minamata disease.
Dioxins: associated with endocrine disruption, mutagenicity, and carcinogenicity.
Persistent organic pollutants (POPs): legacy pesticides and harmful chemicals in plastics and other industrial products can adhere to dust and travel long distances.
Black carbon: fine soot from coal-fired power/coal heating and diesel engines that also travels long distances.
Microplastics and nanoplastics: microplastics have been detected inside atherosclerotic plaques in an Italian study. Uric acid has also been noted in plaques, and hyperuricemia is increasingly linked not only to gout and stones but to vascular, renal, and cardiac harm. Sources include tire-wear particles, textile fibers, and plastic products weathered by UV, wind, and rain into airborne dust.
PFAS (e.g., PFOS/PFOA, FTOHs): per- and polyfluoroalkyl substances and fluorotelomer alcohols can travel far in air, then deposit via precipitation or dry deposition.

Health impacts

In 2024, signal was hard to read amid nonstop infections. In 2025, after cedar and cypress pollen season, we increasingly saw chronic cough, upper-airway pain, summer colds, and asthma exacerbations. Heat alone stresses the respiratory system; the term “desert lung” has even been used in desert regions. Both very high and very low humidity burden the airways.

Worsening hay fever and the spread of air purifiers appear correlated with China’s desertification and economic growth. Many people likely developed—or saw a surge in—such symptoms over recent decades.

When photochemical smog concentrations rise, mucosal irritation occurs: stinging eyes, tearing, sore throat, and cough. In severe cases, shortness of breath, headache, and nausea can follow. Overall, we should consider mucosal irritation and inflammation, as well as disturbances of immune and allergic responses.

Beyond the respiratory tract

Environmental pollutants do more than damage mucosa. As the list above suggests, some are carcinogenic; others relate to atherosclerosis. Nanoplastics—smaller than microplastics—can traverse mucosa into the body and bloodstream, and sufficiently small, hydrophobic/lipophilic particles can cross cell membranes directly. Larger particles may still enter via endocytosis or membrane proteins. Even without diving that deep, anyone with public-health exposure will know these agents affect living organisms in many ways.

Conclusion

Given ongoing desertification and industrialization in China, environmental and air pollution will likely continue to manifest or worsen health harms among Chinese citizens and downwind populations—North Korea, South Korea, Taiwan, Japan, and beyond—under the prevailing westerlies. These impacts reach non-human life as well. However modest my voice, protecting the global environment is essential work we must carry on.

China’s Air Pollution and Transboundary Environmental Impacts on the Health of People in Japan