Thai health authorities are testing hundreds of Bangkok residents for radiation sickness after a leak at a scrapyard hospitalised five people, three critically.

In all, about 500 residents of the eastern suburb of Samut Prakarn have had blood tests and are now awaiting results.

The scrapyard had bought the material from a rubbish collector who had found it in a suburban carpark.

The afflicted workers have suffered blisters, radiation burns and hair loss.

They have become vulnerable to infections because of a drop in their level of white blood cells, the Health Ministry said.

The authorities said they found three canisters containing the radioactive material.

Isotope

The radiation came from the radioactive isotope Cobalt 60, which had been inside a container sold to the scrapyard.

Cobalt 60 is used to produce high-energy radiation for cancer treatments.

It is also used for the sterilisation or preservation of food.

Soomjit Saejia, the scrapyard owner, told doctors that her workers had dismantled a small metal cylinder earlier this month to get its steel outer case.

The material inside gave off a terrible smell, but no one suspected it was radioactive until the man who took it apart fell ill and three dogs at the scrapyard died.

The cylinder has been now located. It reportedly bears the atomic symbol and the words "Atomic Energy of Canada Ltd".

Authorities were hopeful that it would enable them to find the owner - probably a hospital - where it would have been used in an X-ray machine.

Cobalt 60 is a radioactive isotope used in the production of gamma rays and for sterilisation in the food industry.

Cotton gloves

Early on Sunday, officials from Thailand's Office of Atomic Energy for Peace found a small pellet of the isotope at the scrapyard and stored it in a lead-lined container, rendering the area safe.

It is Thailand's first known radioactive leak.

Front page pictures in the Thai press showed officials searching through scrap metal heaps for radioactive waste using sticks and wearing cotton gardening gloves and cloth face-masks.

No more people have so far been reported sick.