A smoky haze caused by forest fires is hovering over parts of Malaysia, posing health and environmental problems.

Air pollution is a problem throughout the region as rapidly developing nations, especially China, invest in industrial and transport infrastructure.

According to environmentalists, the three largest sources of pollutants are power stations, industrial facilities and vehicles. Here are the main pollutants and their effects on the human body.

Particulate pollutants

Although few of the tiny fragments of soot thrown into the air by burning find their way into the lungs - specially adapted cells in our mouth, nose and throat filter out the larger particles - some heavy metals produced in industrial processes do.

They tend to upset the chemical balances in our bodies that prevent oxygen - which would be poisonous to us without the presence of certain compounds - from corroding the cells of our organs.

Other heavy metals cause cancers and can even damage our genetic make-up by attacking DNA.

"These toxic compounds, especially released through the burning of coal, can accumulate in the body, affecting brain and kidney function and cause other organ failures," says Edwin Lau from Friends of the Earth.

Ozone

Not the ozone found naturally in the atmosphere, but the hazy brown smog created by the interaction of sunlight and noxious pollutants such as nitrous oxides, sulphur dioxide and others produced by vehicles and industrial activity.

According to Hong Kong Chinese University's community health professor Wong Chit-ming, ozone attacks the protective cells that line the membranes in the lungs that absorb life-giving oxygen.

"Sulphur is an acid and it corrodes the lungs," says Wong. "It scalds and scars the lung tissue, and reduces your ability to absorb oxygen."

Carbon Monoxide

Produced predominantly by vehicular exhausts, carbon monoxide is a clear odourless poison that the lungs find easier to absorb than oxygen.

"The blood then carries this poison to the organs, consequently starving them of oxygen," says Wong. "They basically die."