LONGMONT -- Barring rain or high winds, Longmont's mosquito contractor is set to proceed with an emergency citywide spraying operation tonight.

Colorado Mosquito Control Inc., which planned to field as many as 9 or more trucks equipped with ultra-low-volume fogging devices, was expected to begin the spraying at about 9 p.m., starting with neighborhoods within city limits but on the inside edges of the city's boundaries.

The spraying was expected to be completed by 2 a.m. Saturday.

Dan Wolford, the city's land program administrator, said accommodations are being made to delay the trucks showing up in downtown Longmont before 10 o'clock tonight. This month's downtown ArtWalk is scheduled for 6:30 to 9 p.m., and post-ArtWalk cleanup work is expected to last until about 10.

Wolford said spraying will also be delayed for the city streets near Garden Acres Park, 2058 Spencer St., until after a softball tournament there concludes at about 10:30 tonight.

If rain, wind or other weather conditions arise that would reduce the effectiveness of the citywide spraying -- intended to kill as many West Nile virus-carrying mosquitoes inside Longmont as possible -- the effort is to be postponed to Saturday night.

A second follow-up citywide spraying effort has tentatively been scheduled for 9 p.m. Monday through 2 a.m. Tuesday. The decision whether to proceed with that Monday night spraying will depend on the number of still-alive mosquitoes captured on Sunday and collected from traps at 11 locations throughout Longmont.

Boulder County, meanwhile, had its mosquito control contractor, OtterTail Environmental Inc., proceed with the Thursday night spraying of the Boulder County Fairgrounds northeast of Hover Street and Nelson Road, as well as about a one-mile-wide buffer of unincorporated areas outside Longmont's southern, western and northern boundaries.

Both the county's and the city's contractors are spraying AquaLuer 20 20, a permethrin-based pesticide. City officials have said that while permethrin poses minimal threat to humans and animals -- at the levels being sprayed and the fogging process used to spray it -- that people can take additional precautions by staying indoors while the spraying is under way.

The city has advised people to close their windows and turn off window-unit air conditioners when spraying is taking place in their neighborhood. They've said people shouldn't let children play near or behind the truck-mounted applicators, and to consult their local physicians for additional precautions if those people suffer from chemical sensitivities or feel spraying might aggravate a pre-existing health condition.

Earlier this week, based on the results of the past two weeks of sampling mosquitoes captured in Longmont traps, Boulder County Public Health recommended that the city conduct the emergency spraying operation.

A number of Longmont residents have raised concerns about the spraying and the impact the chemicals in the pesticide could have on health of people, pets, honeybees, other insects and animals, and the effects on the environment.

Chana Goussetis, a spokeswoman for the county health department, said in an email this morning that "we understand the concern about mosquito spraying and take the decision to spray very seriously. Mosquito spraying uses an ultra-low-volume mist at levels set to be toxic to mosquitoes and not humans."

County officials said more information about West Nile virus, mosquito control, and the testing and safety of the pesticide being used is available online through links on BoulderCountyMosquito.net.

John Fryar can be reached at 303-684-5211 or jfryar@times-call.com.