FAQ - Health Effects of Aerial Spraying for West Nile Virus (WNV) Control

​Why would aerial spraying for mosquitoes be done? 

The decision to reduce the number of adult mosquitoes that can transmit West Nile Virus to humans is based on the recent past where in 2005 we had a rapid increase in human cases of serious WNV illness and the high frequency of finding WNV in local mosquito samples. Without more aggressive mosquito control, we can expect a significant number of people to become seriously ill with WNV infection. The determination for if and when aerial spraying will occur will be communicated to the public. 

What pesticide is used? How safe is it?

Pyrethrins are an insecticide that is used in many lice medications and has been applied directly to humans.  The application, if needed, will be using pyrethrins at a dose of .0025 lbs/acre, which is a very low dose and unlikely to result in toxicity for the vast majority of people.    A link to information on the toxicity of pyrethrins is

Toxic Substances Portal - Pyrethrins and Pyrethroids 

Pyrethrins in the environment are broken down very quickly by sunlight.  Depending on the season this would occur within hours in the summer, and within a day or two in the winter. 

Right now, a decision to conduct aerial spraying has not been made, but would be based on the judgment that the risk of people getting severely ill with West Nile virus is higher than the risk caused by exposure to the doses of pesticide being used.

What precautions should be taken by individuals in areas being sprayed? 

Sacramento-Yolo Mosquito Vector Control recommends that in the areas where spraying would occur, people should close their windows.  There is no need for further precautions, but some people in the past have chosen to turn off air conditioners too.
The spraying, if it occurs, would happen between 8 p.m. and 12 a.m. on selected days.

People who are allergic to these compounds or are concerned about other individual health risks should take this information to their doctors and they, together with their doctor, should decide what the best course of action should be, given that the spraying may go on for several days. If indeed it is even found to be needed this year.

Where else can I find information?

Public Information line (916) 874-2000 or 866-319-2001 (toll free)

Sacramento County Public Health Home Page

Latest West Nile Virus Activity in California

Sacramento Mosquito & Vector Control District