The blue liquid fired from police water cannons at the anti-government protesters at the Pathumwan intersection last night has triggered speculation over its chemical nature.

Many of the protesters complained of skin and eye irritations after being hit by the blue-coloured water jets. Some immediately suspected they were hit with tear-gas, causing a hot debate on the possible harms of the substance in the social media.

It was the first time that the indelible dye was used in water jets by crowd control police against street protesters.  

Pol Maj Gen Piya Tavichai, a deputy metropolitan police chief, insisted last night the chemical was not hazardous.  “It was used in accordance with international standard,” he said.

He said the chemical might cause skin irritations which could be easily relieved if washed with water.  

“The chemical was used to stop the protesters and force them to retreat,” he said.

He also denied that tear-gas was used in the dispersal of the crowd last night even though at one point some of the anti-riot policemen were seen wearing gas-masks.

Police sources said it was the first time that such chemical was used in a crowd control situation.   The bright blue pigment is designed to mark out protesters and help authorities track them downafterward.

However, it’s unclear whether any of the chemical-stained protesters were subject to police action.

When similar chemical was used against pro-democracy protesters in Hong Kong last year, it prompted environmental NGO Greenpeace to raise an alarm that it could contain toxic substances.