Rain unlikely to be “extremely toxic” despite dubious warnings [Updated]

The rain set to fall over Northern California today offers help to firefighters and a chance to clear the air in San Francisco as the storm pushes the ugly toxic haze out of our skies this week.

However, some locals see the incoming precipitation as cause for alarm thanks to a viral message making the rounds on social media warning that the pending showers will be “extremely toxic.”

Most variations of the message, which began appearing on Twitter and Facebook on Monday, present a numbered list of claims and instructions:

1) Smoke will be clearing out as a storm system comes in and it rains.

2) THIS RAIN WILL BE EXTREMELY TOXIC!!!

3) DO NOT ALLOW YOUR ANIMALS AND PETS TO GO OUT IN THE RAIN!

4) Remove all standing water to prevent your pets or other animals from drinking toxic rain water.

5) If your animals need to go outside, immediately wash their paws or, if they are covered in water from rolling in the ground or getting rained on, immediately give them a bath and wash it off.

6) HAVE A SET OF OUTSIDE CLOTHES AND CHANGE AS SOON AS YOU GET HOME.

Although the viral misinformation does not specify precisely why the storm will allegedly create “extremely toxic” rain, it appears that those sharing it are concerned about the possibility of acid rain and similar phenomena stemming from the Camp Fire.

According to the Environmental Protection Agency, “Acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents.”

However, California-based debunking site Snopes says that, according to the California Air Resources Board, there’s little reason to worry, noting that the elements that compose acid rain aren’t present in most forest fires and that the toxic materials floating around the Camp Fire haze don’t pose similar danger:

A spokesperson [for the California Air Resource Board] told us via email that the toxic rain claims were dubious due to the migration of particulate matter after the fire: “Emission from homes and landscapes burned have literally gone south with the prevailing winds last week. The smoke we are seeing now comes from vegetative matter still burning and our own anthropogenic stuff.”

The California Air Resource Board also told us it was unlikely that particulate matter could become incorporated into California rain even if it existed in high concentrations in the area of the fire.

Photo by N K

[Update: Dave Clegern, spokesperson for the California Air Resources Board, confirmed comments to Snopes, telling Curbed SF:

“Emission from homes and other structures that burned already dispersed with the prevailing winds last week. The smoke we are seeing now comes from vegetative matter still burning. Large scale weather systems, like the one California expects Wednesday, tend to dilute wildfire smoke to a point where it becomes nontoxic to human health.”]

ABC 10 cites UC Davis Professor Chris Cappa as saying that rain after/during a wildfire “isn’t necessarily toxic.”

ABC 7 meteorologist Mike Nicco predicts that the storm “is going to push most of the unhealthy air away from us before the rain starts to come down” and anticipates little to no danger.

And Kristine Roselius, spokesperson for the Bay Area Air Quality Management District, told the San Francisco Chronicle much the same, adding that any particles still in the air “will be incorporated into the soil” in fairly undramatic fashion.

The incoming storms do pose real potential danger, but mostly in the form of flash floods and mudslides, especially in areas that were recently the site of forest fires.

• Will Rain Be Extremely Toxic? [Snopes]
• What Is Acid Rain? [EPA]
• Will Incoming Rain Be Toxic? [ABC 10]
• Will There Be Acid Rain? [ABC 7]
• Meteorologists Say Not To Worry [Chronicle]