BILLINGS, Mont. (AP) — Huge gaps between air quality sensors in the western U.S. have created blind spots in the warning system for wildfire smoke plumes sweeping North America this summer.

That comes amid growing concern over the potential health impacts for millions of people being exposed to the pollution.

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Government programs to alert the public when smoke becomes unhealthy rely on monitoring stations heavily concentrated around major cities on the West Coast and east of the Mississippi River.

The patchwork system leaves some people unable to determine risks from smoke, including in rural areas where air quality can quickly degrade when fires ignite nearby.

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To determine the most popular national parks in the United States, Stacker compiled data from the National Park Service on the number of recreational visits each site had in 2020. Keep reading to discover the 50 most popular national parks in the United States, in reverse order from #50 to #1. And be sure to check with individuals parks before you visit to find out about ongoing, pandemic-related safety precautions at www.nps.gov/coronavirus.