EDMONTON -- A newly published study says air downwind from a cluster of petrochemical plants northeast of Edmonton contains pollutants at levels equal to some of the world's largest cities.

Other pollutants, including some known to cause cancer, also measured well above normal. And cancer rates linked to those chemicals were found to be higher in communities closest to the so-called Industrial Heartland.

Although scientists don't definitively link the two, one of the report's co-authors said the findings raise concern about the possible long-term effects of exposure to petrochemical emissions.

"We're suggesting a prudent approach -- reduce the carcinogens now as a preventative measure," said Isobel Simpson, a chemist at the University of California Irvine and co-author of the report published online by Atmospheric Environment.

An Alberta government spokeswoman said the report doesn't necessarily reflect real human exposure to the pollutants.

The area, 30 kilometres northeast of Edmonton and adjacent to the town of Fort Saskatchewan, now holds Canada's largest concentration of petrochemical processors. More than 40 companies, including majors such as Shell and Imperial Oil, are spread out over nearly 600 square kilometres.

Simpson said her lab became interested in 2008, when they were invited to participate in an environmental impact study for a project in the region.

"We were surprised at some of the levels of volatile organic compounds we found."

The team returned two years later and, over the course of two days in August, they took highly precise readings of 77 different compounds downwind of petrochemical facilities and compared them with samples taken upwind at an area farm.

They found smog-causing chemicals in the air at levels comparable to -- and occasionally many times higher than -- some of the world's largest cities and industrial complexes. Those include Mexico City, Beijing and Tokyo as well as Houston, home to the largest petrochemical manufacturing centre in the U.S.

"Each time we find the same thing," said Simpson. "We can get ourselves into an industrial plume and see concentrations for some gases that are higher than what we see when we go to megacities and go looking for dirty air."

Some chemicals were up to 6,200 times more concentrated downwind of industry than in farm samples.

"That is not a surprise," she said.

"What's concerning is the number of carcinogens that we measured. What you don't want to see is elevated levels of those carcinogens in these plumes and that's what we saw."

At least 10 chemicals found in the plumes are either known or suspected carcinogens.

Benzene, conclusively linked to blood cancers, was measured at 77 times background levels. Other chemicals were hundreds of times higher.

Researchers went on to find blood cancers in men in the three counties surrounding the heartland to be consistently higher over the years 1997-2006 than for neighbouring counties.

"It was the blood cancers in men that stood out statistically," Simpson said. "That's occurring in the same place as the emissions of chemicals known to cause those exact cancers.

"We are stopping short of saying one caused the other. But it certainly seems plausible."

Nikki Booth of Alberta Environment said the department has reviewed and welcomes the study. But she said staff don't believe its findings are cause for concern.

"The locations where the data was collected do not necessarily represent the locations where people are exposed," she said in an email. "Calls for an immediate reduction in emissions of known carcinogens cannot actually be supported by the data collected in the report."

Staff are also concerned about methods used to collect the samples, she added.

Simpson acknowledges the chemical levels reported in the study remain under the government's exposure guidelines. But she said those guidelines are aimed at short-term exposures -- not long-term low levels.

The study says recent research suggests there is probably no safe exposure level to benzene.

"It's certainly enough for us to recommend reducing the emissions of these known carcinogens," said Simpson, "especially given that the relationship between exposures over long time periods with benzene isn't well understood."