Almost 30 percent of Beijing's malls and supermarkets have an excessively high amount of formaldehyde concentration in the air, posing a potential threat to customers, according to a study released yesterday.

Formaldehyde is a type of chemical that smells similar to eggs and is used widely to produce building materials, such as drywall, and household products. Long-term exposure may cause serious health problems since it is listed as a "probable human carcinogen" by the International Agency for Research on Cancer, which is affiliated with the World Health Organization (WHO).

The Beijing municipal health bureau said the formaldehyde concentration failed to meet standards of indoor air quality in its four-month investigation of air toxics in indoor public areas between May to August.