"Mosquito days" are trending upward in San Francisco
Add Axios as your preferred source to
see more of our stories on Google.
The number of "mosquito days" — hot and humid weather the flying insects crave — has trended upward in San Francisco over the past several decades, according to a new analysis.
Why it matters: Mosquitoes are more than just a nuisance — they're a public health threat, carrying diseases such as malaria, West Nile, Zika and more, Axios' Alex Fitzpatrick and Kavya Beheraj report.
What's happening: San Francisco had 192 mosquito days in 2022, compared to 183 in 1979, according to a report from nonprofit climate news organization Climate Central.
- Climate Central defines a "mosquito day" as one with an average relative humidity of 42% or higher, plus daily high temperatures of between 50-95° F.
- Of note: Rainfall can influence mosquito activity, and the Bay Area saw historic rainfalls this winter. Mosquitoes breed in pools of standing water, common after major storms.
Be smart: San Francisco's health code requires property owners to keep their properties free of the conditions that allow mosquitoes to breed.
- That means draining all standing water from areas like saucers below flower pots, trash containers and anything else that can collect water outside.
- Otherwise, owners may be subject to a range of fines.
The big picture: 71% of the 173 nationwide locations Climate Central analyzed saw an increase in mosquito days between 1979 and 2022, of about 16 days on average. California nabbed three spots in the top 10.
Zoom in: San Francisco saw the second-highest increase in mosquito days during that time frame.
- Santa Maria, California, saw the highest increase in mosquito days — 43.
What they're saying: "What's really causing this is the rise in minimum temperatures," Kaitlyn Trudeau said, a researcher at Climate Central, told KQED. "There are many more days where the minimum temperature in San Francisco is 50 degrees or above."
- She added, "it's the coastal curse."
The intrigue: Some locations — particularly in the South — are actually getting too hot for mosquitoes, the analysis notes.
- They don't thrive in temperatures above 95° F — an increasingly common reading in southern summers.
- The ongoing Texas heat wave has heat indices reaching upwards of 120°F across parts of the Lone Star state, Axios' Andrew Freedman reports.
What's next: Experimental efforts to control mosquito populations by releasing genetically modified versions of the insects into the wild are underway in Florida and elsewhere.
- Those projects, however, are controversial among some locals and skeptics who view them as tampering with the natural ecosystem.
The bottom line: Mosquitos — and the diseases they sometimes carry — are shaping up as one more climate change-induced problem for many city officials to worry about.
- Yes, but: San Francisco's health department already routinely inspects buildings with rental units and their surrounding areas for mosquito activity, as well as investigates all reports of mosquito activity.