If you see a pickup truck driving through your neighborhood, spraying a mist into your yard, you don't need to count yourself paranoid for wondering what the stuff is, what it's for, and what it's likely to do to you.
We spoke to Ronnie Nease, director of environmental health for the Knox County Health Department. "It's called 30 + 30 Biomist. It's a synthesized permethrin, a contact-only chemical." Manufactured by Clarke Mosquito Control, it's a neurotoxin designed to cause near-instant death to some flying insects by scrambling their nervous systems.
Nease says that common mosquito spray was vetted years ago by the Knox County Board of Health, a panel dominated by medical doctors and other health professionals.
"With any chemical, there could be a risk," Nease says. KCHD recommends that people stay inside as the truck passes to avoid the cloud for at least a few minutes.
Biomist, Nease says, "has no residual effect, and it's very specific for mosquitoes and black midges," the latter better known to some of us as "no-see-ums."
Biomist 30 + 30 is one of Clarke's milder applications, according to the company's website: "For Use Outdoors as an Ultra-Low Volume Application to Control Adult Mosquitoes in Residential and Recreational Areas." The name derives from the fact that it's 30 percent permethrin and 30 percent piperonyl butoxide. The latter is not a pesticide in itself but pairs with the permethrin to make it more effective.
It does warn, "harmful" to humans if swallowed or absorbed through the skin and "causes moderate eye irritation."
Other species are less safe. "The pesticide is extremely toxic to aquatic organisms, including fish and invertebrates," it warns, with a further warning about runoff. Unfortunately, it's also "highly toxic to bees exposed to direct treatment on blooming crops or weeds."
The federal Agency for Toxic Substances and Disease Registry makes similar remarks about permethrin in general, that it's unlikely to harm mammals and specifically bears no link to cancer in humans. It degrades within a few weeks, a little more or less depending on whether it lands on plants or soil. But the agency advises that permethrin shouldn't be used around water due to its toxicity to fish--and that it's "highly toxic to bees."
Knox County didn't take it upon themselves to spray insecticides into neighborhoods until about 2000, soon after the first American appearance of the sometimes-deadly West Nile Virus.
The county was originally more aggressive about prophylactic spraying, covering neighborhoods whether they had evidence of West Nile Virus or not, but following later guidelines established by the Centers for Disease Control, are now spraying only selected neighborhoods known to be at risk. In the past week, the county has sprayed sections of South Knoxville, including Stone Road, Vestal, and Island Home; Heron's Point, near Farragut; Dutchtown and Gulf Park, west of Cedar Bluff, and the Robindale Road area, in West Knoxville; and areas along Central Street near Old North.
Only certain mosquitoes carry the virus. Knox County is blessed with 21 mosquito species, and the only known carrier of the virus is the culex mosquito, known for its unusual light-brown color. It's not nearly as conspicuous here as the aggressive Asian tiger mosquito, which reportedly does not carry the virus. Humans infected have about a 99 percent chance of recovery without serious consequence. It's the fact that the remaining 1 percent sometimes come down with deadly or disabling diseases like encephalitis, inflammation of the brain—or meningitis, inflammation of the lining of the brain and spinal cord—that makes the virus, and the culex mosquito, sufficiently scary.
The KCHD has established 14 traps around the county, including the city; nine of them have turned up mosquitoes positive for West Nile Virus. Nease confirms that Knox County has now seen one human case of West Nile Virus, last week. He declined to mention the site, but reports the patient is recovering.
Some are concerned about health effects of the spray, but others might wonder whether it's enough to make any difference. Nease admits the cloud of Biomist just affects residents' front yards. "The spray on the streets is not getting into your backyard," he says. "It reaches only 25-40 feet each way from the center of the road."
Where using an old service alley is an option, Nease says, they've taken that opportunity, but backyard mosquitoes won't be much bothered by street spraying.
The Dallas, Texas, area, where at least 26 have died this year as a result of the mosquito-borne virus, has resorted to aerial spraying to cover areas more thoroughly. Nease says that wouldn't happen here absent an epidemic, unlikely at this point—and that it would have to be approved well above his office, at the state level.
However, Nease says street spraying has resulted in measurable benefits. "We do notice a reduction in population in the places where we have traps," he says. He doesn't have figures handy, but reports "sometimes large, sometimes small reductions" of mosquito populations after street spraying.
"Mosquitoes are not an exact science," Nease says. "I wish they were."
The first hard frost, perhaps next month, should put an end to this season's concerns.