Health Canada has told the City of Winnipeg to stop using its stock of malathion to fog for nuisance mosquitoes because the chemical is too old.

When fogging trucks sprayed malathion across the city in June, they used insecticide that was at least 13 years old.

The malathion used was part of a batch of 670 litres of second-hand malathion bought by the city from the Saskatchewan government. The province had purchased the chemical in 2003 and no longer wanted it.

But the label for the product says it must be stored for no longer than one year.

The City of Winnipeg fogged with malathion in June. (CBC) "Health Canada has advised the City of Winnipeg that all future use of this product must be in accordance with the label," a Health Canada spokesperson told CBC News.

The federal regulator says data submitted in a 2012 review of malathion showed the concentration of isomalathion, an impurity that is more toxic than malathion, increases with temperature and time.

The city's other malathion inventory is also older than one year -- more than 1,000 litres purchased in 2009 and a 2,000-litre batch from 2007 that the city got from the Manitoba government.

As a result, the federal regulator took compliance action against the city under the Pest Control Products Act of Canada.

City didn't know of label change

City officials say the mistake was inadvertent.

"We weren't aware that there was a change in the label in terms of the storage conditions," said Ken Nawolsky, superintendent of the city's insect control branch. "Actually, it was something that astounded me."

The city wasn't aware of changes to the federal rules for storing malathion, said Ken Nawolsky, superintendent of Winnipeg's insect control branch. The city only became aware of the one-year storage limit when contacted by the CBC I-Team, he said.

Label is a term used by Health Canada for the multi-page document containing the official regulatory information about the product. It is distributed with regulated pesticide products themselves and appears on the regulator's website.

Health Canada said the malathion label was changed in 2014.

"It's unfortunate that this change happened without our knowledge but it's still a fact of due diligence on the part of the city to further read the label to see if there was a change to the label," Nawolsky said.

He added that there should be a better way of informing municipalities about changes in the rules.

Nawolsky vowed that the city will improve its reviews of product label changes.

"We need to know about it, so that we can take the appropriate action to ensure the safety and ensure that we are in compliance with the product specifications," he said.

Mosquito fogging is currently on hold while the city awaits word from Health Canada's Pest Management Regulatory Agency on whether any of the older malathion inventory can be used.

Product degrades over time

The city did follow the other label requirements, such as storing the malathion in a cool, dry, well-ventilated place and testing it for purity and efficacy.

According to malathion manufacturer Cheminova, the chemical degrades over time but that degradation depends on time and temperature. That "would be hard to calculate in this case," said Cheminova, which is owned by FMC Agricultural Solutions.

"However, it's likely the product is still effective against mosquitoes even after this amount of time," the company said.

Nawolsky believes that based on how the city stored its inventory of the insecticide, there wouldn't be any degradation.

"We're very grateful that this was brought to our attention so that we can address this, but the important part is that the city in fact has been doing more than adequate basis in terms of ensuring that … by regularly testing it, storing it in proper storage facilities," he said.

The Saskatchewan government has said the 2003 malathion was continuously stored in an approved facility while in that province.

Even though it can't use the malathion, Winnipeg is now applying more rigorous testing to the product it has in stock.

The city has sent samples of its malathion to an Ontario laboratory to test for isomalathion, the impurity that can increase at elevated temperatures or for extended periods of time. It previously had not been testing for the toxic substance.

Expiry dates matter

University of Manitoba Soil Science professor Annemieke Farenhorst says expiry dates on chemicals make sense.

"There's an expiry date on everything, when we go to the grocery store and we look at an expiry date for milk," she said. "The same holds for pesticide products so manufacturers usually provide a certain time that you should use the product."

"Going beyond the expiry date is not advisable," said University of Manitoba soil science professor Annemieke Farenhorst. (CBC) Farenhorst says expiry dates are an important message from manufacturers on how long they can guarantee the product will work.

"So going beyond the expiry date is not advisable," she said.

Winnipeg's dated product also raises questions about malathion stock in other Manitoba municipalities.

In the last two years, the province has handed out 33 permits for malathion fogging to communities and events outside of Winnipeg.

At this time, it's not clear whether communities in Manitoba will be allowed to use the outdated malathion if a mosquito-borne public health threat like West Nile Virus is detected.

Manitoba says there's been no West Nile Virus found in the province this year.

In 2015, malathion was classified by the World Health Organization's cancer research agency as "probably carcinogenic to humans."

Later this summer, the U.S. Environmental Protection Agency (EPA) is expected to release a draft risk assessment report that will look at the effects of malathion on human health, including the cancer classification.

An EPA spokesperson said the agency is also analyzing "the nation-wide effects of malathion on endangered and threatened species and designated critical habitat."

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