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Officials Urge Indoor Stay, MERV-13 Filters Combat Smoke Heat

7/20/2026

Summary

Story summary

U.S. health officials advise the public to stay indoors and use high-efficiency filtration.
The University of Wisconsin-Milwaukee recommends replacing HVAC filters with MERV 13 or higher and operating systems in recirculate mode.
The Environmental Protection Agency says DIY box-fan filters built with four 20-by-20-inch MERV 13 furnace filters perform as well as commercial purifiers.
Researchers warn that heat plus smoke increase respiratory symptoms and recommend keeping indoor temperatures cool.

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Story summary

U.S. health officials advise the public to stay indoors and use high-efficiency filtration.
The University of Wisconsin-Milwaukee recommends replacing HVAC filters with MERV 13 or higher and operating systems in recirculate mode.
The Environmental Protection Agency says DIY box-fan filters built with four 20-by-20-inch MERV 13 furnace filters perform as well as commercial purifiers.
Researchers warn that heat plus smoke increase respiratory symptoms and recommend keeping indoor temperatures cool.