To Spray or Not To Spray

Commentary, Environment, Rolf Penner

Mosquitoes are as common to the Winnipeg experience as the wind chill in January and the blazing prairie heat radiating off the concrete in August. None of which are considered pleasant when experienced at any level beyond moderate. In all three cases, technology has been our ally by making life far more pleasant than it ever was for people who originally settled here.

Modern heating and air conditioning—not to mention the slurpee—have mostly transformed the weather from a real threat into a topic of conversation. Similarly, since the late 1950s, we have counted on malathion to rid the city of the swarms of unwelcome, bloodsucking, barbeque-wrecking party crashers known as ”skeeters,” when their numbers got too high.

Alas, as with many cheap, safe and effective enhancements of modern life, malathion has become a target of fear mongers and their associates, the politically correct. In their efforts not to offend anyone, public officials have somewhat bought into anti-chemical propaganda. They have repeatedly assured us that the only reason we are using malathion this year is the very real threat of West Nile virus, which caused two deaths in 2003. That the risk from West Nile is greater than the risk from malathion. Which implies that spraying malathion to control mosquitoes involves some kind of risk to the public. But does it?

According to the data from Canada’s Pest Management Regulatory Agency (PMRA), the branch of Health Canada responsible for safety assessments of all pesticides before public use, it doesn’t appear that there is any risk whatsoever. PMRA scientists are reluctant to give absolute assurances. They prefer to hedge their bets by saying that, “large-scale applications of malathion in residential areas for control of adult mosquitoes do not pose an unacceptable risk to bystanders.” But a review of the pertinent studies makes it hard to conclude that it is anything other than perfectly safe.

The most important rule of toxicology is that it is “the dose that makes the poison.” The maximum allowable rate of application for malathion is 6.08 milligrams of active ingredient per square metre for ground spraying and 26 mg. per square metre for aerial applications. These rates have a safety factor built into them of 300 to 1000 times below the “no observed adverse effect level” derived from animal studies. In fact, one such study could find no symptoms in rats that were fed 25 mg. per kg. of body weight for two years. Another dermal study showed that the lethal dose of active ingredient required to kill 50% of the experimental animals was over 4000 mg. per kg. of body weight.

Malathion is considered so safe that human trials have also taken place. Workers exposed to concentrations of up to 85 mg. per cubic metre for one hour over 42 days suffered no adverse effects. Another test involved a shirtless 70 kg. ”jogger” in short pants standing in the direct path of the aerosol generators on the back of a spray truck. This worst case scenario resulted in the total deposition of 1.1 mg. of product per kg. of body weight. Since the maximum absorption rate through the skin has been found to be 10%, the actual exposure level to this person was .11mg. per kg. of body weight.

The “dose to poison” relationship is often described this way: one aspirin and your headache goes away; one bottle of aspirin and you go away. There is no conceivable way that at the levels being sprayed that even the smallest, most sensitive members of the community, or their pets for that matter, could come remotely close to the exposure levels required (the entire bottle of aspirin) to cause harm. In addition to dispelling heath concerns, these facts also neutralize the property rights argument of those who don’t want their yards sprayed. For it to be considered a violation or even a nuisance, some kind of harm must be done to the property which prevents one from enjoying it.

West Nile or not, malathion in and of itself when properly used is no threat to the community. If it were, it would not be registered for large-scale spraying applications in suburbia, period. To imply otherwise does not play straight with the public. Public authorities should make its safety clear.

So why not spray? A cold slurpee on a hot summer day is a wonderful thing. Enjoying it outdoors on a park bench without getting swarmed by mosquitoes is a slice of heaven. A slice that is safe.