People who queued to buy a dehumidifier in anticipation of a foggy, humid spring should probably have bought an air purifier instead. The year's first quarter has been unusually bright and dry and scientists are looking into the possibility that this is why the air was also unusually dirty.
This spring has been one of the most polluted on record, especially on the roadside.
The air pollution index was at a very high level for about a third of the January-March period, almost three times longer than during the same period last year when dust from a mainland sandstorm hit the city.
Air quality was worst in Central, where 912 hours of very high readings were registered in the quarter. This was followed by 808 hours in Causeway Bay and 488 in Mong Kok.
A very high level reading of 100 or more means at least one of the air pollutants being monitored has surpassed the air quality objectives set by the Environmental Protection Department in 1987. Exposure to bad air pollution can cause or aggravate respiratory problems or heart disease.
Another indicator of air pollution, reduced visibility, also set record highs in the first quarter. The number of hours with visibility falling to below 8 kilometres, excluding days with high humidity and fog, surged to 190 and 207 hours in February and March, the highest ever for each of the two months, according to the Hong Kong Observatory.
Figures for pollutant concentrations, separate from the air pollution index, show roadside nitrogen dioxide levels surged by 21 per cent.
The department blamed less rain and stronger sunshine for the increased pollution. "The weather was in general dry with rainfall low," a spokesman said. "Air pollutants would thus stay in the atmosphere instead of being washed out by the rain. In addition, there was less cloud cover than normal, which caused solar radiation that promoted photochemical smog formation."
But Professor Alexis Lau Kai-hon, an atmospheric scientist from the University of Science and Technology, said vehicle emissions should take most of the blame.
"I did not notice any major changes in regional pollution and this no doubt suggests the air quality deterioration by the roadside is of our own making," he said. "Year after year, the city's vehicle fleet is ageing and will emit more pollution. But we do not have any system, such as thorough vehicle maintenance and repair, to monitor the changes."
Professor Wong Tao, a Polytechnic University air pollution specialist, said the weather, still dominated more by air from the north than that from the ocean in the south, definitely contributed to poor air quality.
But he believed more vehicle use in the improving economy and poor air circulation in a city dominated by high-rise buildings were also factors in pushing up roadside pollution.
"Although it is officially spring now, the weather actually looks like autumn where it is drier and cooler. The lack of rain also meant that pollutants in the air could not be washed to the ground," he said.
Observatory figures show last month was the driest March ever, with rainfall about a third of normal.
Professor Johnny Chan Chung-leung, a veteran weather scientist at City University's school of energy and environment, also believes the unusual weather had something to do with the pollution. "I don't think there has been a substantial increase in emissions so meteorological conditions might be at play," he said.
Chan said weaker northerly winds might not be conducive to dispersion of air pollution, while the cooler-than-usual weather might create more frequent inversion layers - warm air above cool air - that also block dispersion of pollutants.
A spokesman for the department said changes in air quality should be measured on a long-term basis, and pollutant concentrations were more reliable indicators than number of hours at an air pollution index level.
From 1999 to last year, the concentration of respirable suspended particles fell by 34 per cent and sulphur dioxide by 63 per cent. But nitrogen dioxide surged by 18 per cent in the same period.