Air quality in Bulgaria has deteriorated sharply over the past week with dust pollution up to five times over the norm, posing a grave health risk, the environment ministry said Wednesday.

"The norms for pollution with fine dust particles have been surpassed since November 15," Valery Serafimov, an air pollution control official at the Bulgarian Environmental Protection Agency, which is part of the ministry, told 24 Hours newspaper.

The concentration of fine dust particles in the air was over five times the norm of 50 micrograms per cubic metre in the western town of Pernik on Monday and four times in the capital Sofia.

"This pollution is typical for winter days without wind. It is due to the massive use of coal for heating," both in thermal plants and by individual households, Serafimov said.

The latest UNDP Human Development Report issued on November 2 found that Bulgaria, along with Armenia and Romania "lead the world in deaths from outdoor air pollution".

"Long-term exposure to outdoor air pollution causes respiratory disorders, immune system damage and carbon monoxide poisoning, among other deleterious effects," it said, citing industrial pollution as a serious health risk.

A Bulgarian doctor cited by 24 Hours also warned of a heightened risk of heart attack and stroke.

Bulgaria's latest census in February found that 57 percent of all households burn wood and coal for heating, which is much cheaper than electricity or central heating.

Only 13.7 percent of households had access to central heating, while gas use was limited to a mere two percent.

Bulgaria's heating plants, which run on coal or fuel oil, old second-hand vehicles without catalytic converters and dusty roads that are rarely washed also contribute to urban smog, a recent environment ministry report said.