Toronto, July 16 (IANS): The expensive air filter that you installed in your house in the hope of getting purer air could actually be emitting poisonous gases, a study says.

Concordia University researchers have found this is happening with a type of air filter called photocatalytic oxidation (PCO), a product already in the market.

The filter's chemical by-product is Formaldehyde - a known human carcinogen, found the study published in the journal Building and Environment.

"Some of the gases to come through the system are more dangerous than the original gas," said professor Fariborz Haghighat Concordia University research chair of energy and environment.

With countries like China and Korea eager to fix growing air pollution problems, engineers and consumers desperate for new technologies have been forced to try to evaluate and compare PCO systems themselves in the absence of standards.

The battle against chemical contaminants is a challenge because gases come from so many sources - carpets, paint, treated wood, and then there are all the perfumed products we use individually.

"Sometimes the concentration of gases inside the building is almost 10 times more than outside," Haghighat said.

While regulations limit percentages of chemicals in the air, there are fewer governing specific technologies.

"Because it's new technology and the public accepts whatever the industry says, if a company says it is working they believe it. But it has not been tested by a standards organisation. Usually that takes time, 10 to 15 years," he said.

The PCO system currently in use employs mechanical ventilation, where fresh air is pumped in from outside and taken out from inside, the method is not very energy efficient.