Malathion sprayed across Winnipeg in June may not have met World Health Organization (WHO) guidelines, according to an analysis of lab results.

The City of Winnipeg sent samples from two of its batches of the mosquito-killing insecticide to a lab for analysis after a CBC I-Team story revealed the city's supply had been stored too long to be in compliance with federal rules.

Toxic isomalathion levels can increase in the chemical with time or when exposed to heat. The City of Winnipeg's supply was years-old with some product dating back 13 years.

An analysis of lab test results revealed that a 2,000-litre batch of malathion from 2007 obtained from the province does not meet WHO guidelines because of elevated levels of isomalathion. The cut off for isomalathion concentrations in any given malathion batch is 0.4 per cent.

The analysis report by Elias Consulting notes that the 2007 batch has "excessive isomalathion levels and the toxicity could be 2-4 times the level" of the WHO guildelines.

"For the public, this would be the equivalent of applying compliant malathion at up to four times the concentration," the report notes.

The city says it is in talks with the province to return its stock of 2007 malathion after lab tests showed it contains unacceptable levels of isomalathion. (Tarek Mahmud/Flickr)

None of the 2007 product has been used according to the city, but earlier this summer the city depleted its supply of an even older batch of malathion dating back to 2003. None of the 2003 stock had been tested for isomalathion levels, but according to the lab analysis, "it should be assumed" that had it been tested isomalathion toxicity levels would also likely have been higher than the allowable limit.

The city says based on the report, "public exposure to isomalathion would likely be below the Acute Exposure Guideline Levels for Hazardous Substances (AEGL) and would not present an unacceptable risk to the public or to workers."

The city says it is currently in discussions with the province to return the unusable 2007 malathion.

The city's 2009 batch of malathion did meet WHO and AEGL standards. The city has restarted its fogging program with that product.

Even though federal rules state malathion cannot be stored longer than one year, Health Canada's Pest Management Regulatory Agency will allow the city to fog with older product so long as it is tested for toxic isomalathion by an accredited lab.