One of the researchers said indoor air pollution - from perfumes, cleaning products and cooking fumes - turned the average Hong Kong home into a “mini chemical warehouse”.

Studies in other countries have also found indoor air pollution is higher than that outdoors. The situation in Hong Kong could be aggravated by factors such as high-rise living, subdivided flats, a lack of windows and ventilation in some rooms, and restaurants occupying ground-floor premises in residential buildings, one of the researchers said.

The research was conducted by Baptist University’s biology department and household appliances manufacturer Dyson. Air quality was tested in 32 homes, with samples taken in their living rooms and kitchens, and directly outside these rooms.

The tests showed the median level of indoor VOCs in most of the homes tested was 345 parts per billion (ppb) compared with 95.5ppb outside the flats. Hong Kong’s Environmental Protection Department says VOCs should not exceed 200ppb in offices. Kitchen VOC levels were far higher.

“VOCs are consistently higher indoors than outdoors. Even so, the research found that the kitchen VOCs are on average 1,258 per cent higher than outdoors,” said Baptist University’s Dr Lai Ka-man, who led the research in March.

The researchers’ report, citing the US US Environmental Protection Agency, says: “Toxic fumes released from cleaning solvents, deodorants and scented candles are some of the most common indoor air pollutants. Other major indoor air pollutants include gases from cooking, mould, pet hair, pollen and allergens.”

The scientists tested for two classes of particulate matter (PM), fine particles resulting from combustion such as that by vehicle engines and power stations. In the home, particulate matter can be produced by cooking and by smoking tobacco. Exposure to excessive levels can lead to allergic reactions of the lower respiratory tract, such as asthma, and to strokes and heart attacks, according to health professionals.

Readings were also taken at seven outdoor locations around the city.

The tests found micro-particle pollution, or PM0.1, was on average 68.5 per cent higher inside homes than outside. (PM0.1 is one-thousandth the diameter of a human hair, and can penetrate the lungs and enter the bloodstream.)

“The highest outdoor PM0.1 was 94 million particles per litre (ppl), recorded at the junction near the Sogo department store in Causeway Bay. The highest indoor sample was a worrying 46.8 per cent higher, at 138 million ppl [recorded in a Tai Wo flat in the New Territories]”, Lai said.

Hong Kong has no guidelines on recommended levels of indoor particulate matter, but it is indicative that outdoor readings in busy roadside areas often reach dangerous levels on the air pollution index – levels that, ironically, trigger warnings to residents with respiratory problems to stay in their homes.

Surprisingly, Lai said, indoor levels of PM2.5 were higher than outdoors. Indoor readings were on average 9 per cent higher than the highest reading recorded outdoors, of 369 ppl, outside Sogo in Causeway Bay; the PM2.5 reading in Nathan Road, Mong Kok, one of the city’s busiest urban roads, was 363ppl.