Hong Kong experienced a marked decline in air quality over the past two days, prompting authorities to issue health warnings due to elevated pollution levels. Several roadside monitoring stations registered significant increases in ozone concentrations, with some locations reaching "serious" health risk levels, prompting experts to advise vulnerable populations to stay indoors until conditions improve.
On Wednesday afternoon, the Air Quality Health Index (AQHI) managed by the Environmental Protection Department recorded a "serious" health risk rating at the Tuen Mun monitoring station, maintaining this level for two consecutive hourly readings. The elevated pollution persisted through Thursday, with stations in Central/Western, Southern, Tsuen Wan, Tuen Mun, and Sha Tin reporting "high" health risk levels around 4 p.m. The Southern station escalated to a "very high" level by 5 p.m. before pollutant concentrations gradually declined in the evening.
Experts attributed the surge primarily to elevated ozone levels driven by photochemical smog formation. Ho Kin-fai, a professor at the Chinese University of Hong Kong’s JC School of Public Health and Primary Care, explained that stagnant light winds combined with strong sunlight and high temperatures had intensified smog activity, contributing to the spike in ozone. He cautioned that these conditions were expected to persist for several more days.
The AQHI, introduced in 2013, assesses short-term health risks associated with air pollution by analysing three-hour average concentrations of ozone, nitrogen dioxide, sulfur dioxide, and particulate matter (PM2.5 and PM10). The index ranges from 1 to 10+, with higher readings indicating greater health concerns. The Environmental Protection Department advises children, the elderly, and individuals with heart or respiratory conditions to minimize outdoor activities when the index reaches the "high" level and to remain indoors if it attains the "serious" level.
Leung Wing-mo, a former assistant director at the Hong Kong Observatory, noted that air quality issues tend to worsen in winter, largely due to meteorological factors such as the northeast monsoon, which can transport pollutants from mainland China into the region. However, Leung highlighted that air pollution levels in the Pearl River Delta have shown improvement over the past two decades.
The recent pollution event underscores ongoing challenges in managing air quality in Hong Kong, especially during periods of stagnant weather and strong solar radiation conducive to ozone formation. Authorities continue to monitor conditions and provide public health guidance as necessary.