Poo, pesticides and nicotine are being inhaled by smokers of nicotine-free electronic cigarettes, a study has found.

Researchers analysed the liquid in e-cigarettes sold online and over-the-counter as nicotine-free and found 60 per cent contained the highly addictive stimulant.

All held traces of a toxic chemical - 2-chlorophenol - commonly used in insecticides, herbicides and disinfectants.

Head researcher Alexander Larcombe said he was surprised to find the toxin as it is known to irritate human airways and skin.

"It's classified as acutely toxic," Professor Larcombe said.

It is illegal in Australia to sell e-liquids which contain nicotine.

But a third of the study's samples held nicotine levels similar to vaping a low-dose nicotine e-cigarette.

It is illegal to sell e-liquids which contain nicotine in Australia - but a third of a study's samples contained traces. (AAP)

"We also found other things - by-products of animal or human bodily functions - which indicates the process of making the e-liquids might not be as clean as you might hope," he said.

Ingredients commonly used to make flavouring, soaps, detergents and solvents were also found.

Prof Larcombe said most were benign food additives but their safety, when heated between 200 to 250 degrees and inhaled, was unknown.

"The real take-home message here is that people are buying these liquids unaware of what's in them, and there's no way they can tell based on what the label says," he said.

The study, published in the Medical Journal of Australia, was a collaboration between Telethon Kids Institute, Curtin University and The University of Western Australia.