Weerawan Taengkaew said the gels, which contain more than one per cent of methanol, could cause blurred eyesight and headaches.

"The fumes could lead to a loss of visibility and be fatal if inhaled in large amounts," she said.

She encouraged food shops and restaurants to use only ethanol-based alcohol gels to reheat food on tables.

"Customers are already aware of alcohol gels when the fumes irritate their eyes," she said.

Owners and operators of restaurants seeking safe methanol-based alcohol gels should look at the Thai Industrial Standards Institute (TISI) labels on the containers, which also carry the names of the manufacturers, to make sure they are using safe gels.

The FDA deputy chief also encouraged operators to lodge a petition to the FDA - at call centre 1556 - if they suspect the gels they use are unsafe, otherwise restaurant owners could find themselves subject to criminal penalties.

Test kits to determine whether the gels are safe are available to restaurants owners for Bt200 apiece. Each kit can be used for 10 tests.

An FDA study last year found that 37 per cent of alcohol gels were unsafe, compared with 13 per cent in 2004.

As of April this year, 18 per cent of gels tested by the FDA were found to be unsafe.

Criminal action has so far been taken against two manufacturers of the offending gels.

There are 20 manufacturers of safe ethanol-based alcohol gels that are certified by the TISI.