PARIS, France -- Remnants of industrial chemicals in the air can potentially transform into new substances more toxic and persistent than the original pollution, according to a global study published on Wednesday.

Using samples gathered around the world, the study published in Nature found that these previously unidentified products are present in the atmospheres of 18 big cities including Lagos, New York, Tokyo and Warsaw.

Regulatory guidelines like those listed in the Stockholm Convention assess the danger of different chemical pollutants based on how long they remain in the environment, how toxic they are and to what degree they contaminate living things. But, the study notes, this approach has been limited to a list of known substances and does not take into account how they may change as they break down.

The research proposes a new framework using laboratory tests and computer simulation to predict what chemicals will arise as products interact with the air and how toxic they will be.

Study main author John Liggio, a research scientist for Environment Canada, worked with a team to test the framework on nine flame-retardant chemicals most commonly found in the atmosphere. "They are chemicals that are added to a large variety of materials to delay the onset of fire," Liggio told AFP. In a laboratory, they observed how these chemicals changed over time when in contact with oxidants in the air and found that they gave rise to 186 different substances.

Comparing these new substances with field samples, they found 19 derived from the five most common flame retardants. None of the 19 had ever been identified in the ambient atmosphere before. The team then used computer simulations to gauge the persistence, toxicity and bio-accumulation of the derived chemicals. They discovered that the new chemicals could have longer-lasting impacts on the environment and could be more toxic than their parent chemicals -- in some cases 10 times as much.

"The framework should provide a new avenue for including transformation products in routine air-monitoring programmes and for prioritising transformation products of high concern for further scrutiny," the study says.

While the study looked at 9 common chemical pollutants and their 19 daughter chemicals present in the urban air samples, Liggio says these results are only the tip of the iceberg.

"Likely thousands of different chemicals exist," he said, adding that future tests will look at vehicle tire chemicals, antioxidants, and other plastic additives.

Another goal is to test toxicity of the pollutants in real-life studies, going beyond the computer modelling used for this study.