A Reuters investigation has found that a government network of 3,900 monitoring devices across the country routinely missed major toxic releases and daily pollution dangers.

The investigation by journalists Tim McLaughlin, Laila Kearney and Laura Sanicola found that the network of pollution-monitoring devices overseen by the U.S. Environmental Protection Agency but maintained and operated by state and local environmental agencies have missed huge pollution spikes like refinery explosions over the last decade and could be providing inaccurate assessments of daily air quality in polluted areas.

The reporters found that despite thousands of hospitalizations and self-reported toxic emissions, the network found no risks from 10 of the biggest refinery explosions.

They also found that monitors that make up the network are “sparsely and poorly placed,” leading to inaccurate readings that could pose a health risk. Monitor data is used to calculate an area’s Air Quality Index, a metric upon which many Americans depend to guide their daily decisions.

The report found that the number of monitors has declined as state and local environmental agencies cut spending.

You can read more details about those and other findings in their investigation here.