# Why arsenic after wildfires?

**URL:** <https://boards.straightdope.com/t/why-arsenic-after-wildfires/756242>\
**Category:** Factual Questions\
**Created:** [June 1, 2016, 4:53pm UTC](https://boards.straightdope.com/t/why-arsenic-after-wildfires/756242 "2016-06-01T16:53:47Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Canadjun](https://avatars.discourse-cdn.com/v4/letter/c/76d3ee/32.png) [@Canadjun](https://boards.straightdope.com/u/Canadjun)\
**Post date:** [June 1, 2016, 4:53pm UTC](https://boards.straightdope.com/t/why-arsenic-after-wildfires/756242/1 "2016-06-01T16:53:47Z")

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It looks like **most** evacuated residents of Fort McMurray are able to go back over the next few days, but some people in the most severely burned areas can’t re-occupy their homes even if they look fine, because of contamination due to arsenic and various nasty compounds.

> **[Notley says 'safety and health' top priority as return for some Fort McMurray...](https://www.cbc.ca/news/canada/edmonton/hundreds-of-undamaged-fort-mcmurray-homes-declared-unsafe-due-to-toxic-ash-1.3607928)**
>
> With residents set to start returning to Fort McMurray tomorrow, the premier announced Monday that hundreds of undamaged homes in neighbourhoods hardest hit by the wildfire are not safe to live in.

What I do not understand is where did the arsenic come from??? Poisonous **compounds** I can sort of understand, but arsenic is an element - if it is there now it was there before the fire.

So, any suggestions where the arsenic might have come from?

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**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\
**Post date:** [June 1, 2016, 5:06pm UTC](https://boards.straightdope.com/t/why-arsenic-after-wildfires/756242/2 "2016-06-01T17:06:54Z")

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[Chromated copper arsenate](https://en.wikipedia.org/wiki/Chromated_copper_arsenate) is a very common wood preservative. Burning will release the arsenic.

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**Author:** ![TroutMan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troutman/32/6721_2.png) [@TroutMan](https://boards.straightdope.com/u/TroutMan)\
**Post date:** [June 1, 2016, 5:07pm UTC](https://boards.straightdope.com/t/why-arsenic-after-wildfires/756242/3 "2016-06-01T17:07:13Z")

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Arsenic is naturally occurring in soil, but not at the levels found in the ash. Most of it has to be from the burning of man-made materials. Fires of this magnitude burn all the hydrocarbon-based materials, leaving the metals.

The arsenic specifically is probably from treated wood.

_(ETA like **dtilque** said)_

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**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\
**Post date:** [June 1, 2016, 5:12pm UTC](https://boards.straightdope.com/t/why-arsenic-after-wildfires/756242/4 "2016-06-01T17:12:40Z")

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Some may have come from burning wood treated with copper chromated arsenate wood preservative used in decks, porches, and other outside structures. It’s been discontinued in residential use in the US and Canada since 2003, but it’s still allowed for industrial use, and existing lumber is still in use.

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**Author:** ![Canadjun](https://avatars.discourse-cdn.com/v4/letter/c/76d3ee/32.png) [@Canadjun](https://boards.straightdope.com/u/Canadjun)\
**Post date:** [June 1, 2016, 5:13pm UTC](https://boards.straightdope.com/t/why-arsenic-after-wildfires/756242/5 "2016-06-01T17:13:04Z")

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> [@dtilque](#):
>
> [Chromated copper arsenate](https://en.wikipedia.org/wiki/Chromated_copper_arsenate) is a very common wood preservative. Burning will release the arsenic.

Oh! That makes perfect sense. Thanks!