A Canadian coroner has ruled a banned pesticide that kills everything it touches likely caused the deaths of two Canadian sisters in a Thai hotel room in 2012.
The Quebec coroner, Renee Roussel, said 20 western tourists - including New Zealander Sarah Carter - have died under similar circumstances in Asian hotels since 2009.
The Bangkok Post reported Audrey Belanger and Noemie Belanger, who were both in their twenties, died in their beds, covered in vomit, on the island of Koh Phi Phi.
A Thai doctor said at the time the common pesticide DEET was the most likely cause of their deaths but Renee Roussel said it was more likely a highly toxic pesticide called phosphine was to blame.
She said the hotel probably used phosphine - which can kill any organism that breathes - even though it is banned in Thailand.
Loose controls over Thai chemical use, says father
In the case of New Zealander Sarah Carter, in 2011, there was speculation the insecticide chlorpyrifos was involved.
The 23 year-old's two friends, Amanda Eliason and Emma Langlands, also fell ill.
Sarah Carter's father Richard said the coroner's ruling on the deaths of the two Canadian sisters showed Thai authorities still had loose controls over chemical use.
He said Thailand needed to adopt the same chemical use standards as first world countries.
"I would just like to see the Thais take a bit more responsibility and view these cases seriously."
Richard Carter said international pressure needed to be applied to Thailand.