Experts pore over contaminants in Japan's radioactive spill

* Worries abound over radiation found in food, water

* Contaminant caesium can remain for more than 200 years

* Excessive levels of radioactive iodine found

By Tan Ee Lyn

HONG KONG, March 22 (Reuters) - Authorities across Asia stepped up checks this week on Japanese imports after radioactive contaminants showed up in food and water in quake-stricken Japan following blasts at a nuclear plant last week.

Experts are most worried about three radioactive substances -- iodine-131, caesium-134 and caesium-137 -- all of which can cause various types of cancer years later.

Caesium-137 is of particular concern as it can stay in the environment and potentially cause havoc for hundreds of years. It takes 30 years for this contaminant to lose its power by half -- what experts refer to as a "half life".

At this rate, it would take at least 240 years for the contaminant to exhaust all its radioactivity.

"Caesium-137 can last for hundreds of years. If exposed, one can get spasms, involuntary muscular contractions and may lose the ability to walk. It also causes infertility. High doses will also damage a person's DNA and cause cancer later," said Lee Tin-lap, an associate professor at the Chinese University's School of Biomedical Sciences in Hong Kong.