On their website, EnergyStorage.org, American Clean Power addresses this claim:
Past incidents demonstrate that fires are contained within the facility, and air quality in
neighboring areas remains at safe levels.
Laboratory testing of emissions from Li-ion cells in thermal runaway shows that emissions are similar to those found in plastics fires. (DNV-GL, Considerations for ESS Fire Safety, Report for Consolidated Edison and NYSERDA, 2017, https://www.nyserda.ny.gov/-/media/Project/Nyserda/files/Publications/Research/Energy-Storage/20170118-ConEd-NYSERDA-Battery-Testing-Report.pdf)
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During an ESS battery fire, only trace amounts of chemicals are detected in sampling around the event, and overall air quality remains at safe levels.
During a fire at a Tesla Megapack at Moss Landing in California, air-quality testing showed no hazards to human health. (https://www.montereycountynow.com/blogs/news_blog/air-quality-testing-showed-no-hazards-to-human-health-amid-battery-fire-in-moss-landing/article_5a0ee07a-4125-11ed-a797-c31048cab7a5.html)
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