Cities such from Washington, D.C., to Boston have seen significant improvement in air quality.

March 2020 showed the lowest monthly atmospheric nitrogen dioxide levels of any March on record.

However, in the U.S., "these recent improvements in air quality have come at a high cost."

Air pollution has dropped by 30% in the big cities of the Northeast over the past few weeks as the coronavirus pandemic worsened and people stayed home, according to NASA satellite data.

In fact, cities such as Washington, D.C., Philadelphia, New York City and Boston have seen significant improvement in air quality similar to the improvements in Italy and China during prior coronavirus lockdowns, AccuWeather said.

Nitrogen dioxide (NO2), which is primarily emitted from burning fossil fuels for transportation and electricity generation, can be used as an indicator of changes in human activity, according to NASA. With people staying home and not driving their cars, air pollution has declined in a big way.

Other major cities that saw recent major drops in NO2 levels included Cleveland, Detroit, Buffalo, New York, and Pittsburgh, according to AccuWeather.

Though year-to-year variations in weather can cause variations in the monthly averages for individual years, in the Northeast, March 2020 showed the lowest monthly atmospheric nitrogen dioxide levels of any March on record, which spans the past 15 years.

Last month, pollution reductions were also noted in countries hit hard by the coronavirus, including China and Italy.

This led to speculation that the virus had actually saved more lives than it took. According to Paul Monks of the University of Leicester, "the World Health Organization (WHO) estimates that about 3 million people die each year from ailments caused by air pollution and that more than 80% of people living in urban areas are exposed to air-quality levels that exceed safe limits."

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Stanford University's Marshall Burke also noted in March that "the reductions in air pollution in China caused by this economic disruption likely saved 20 times more lives in China than have currently been lost due to infection with the virus in that country."

However, in the U.S., the trade-off has been steep. "These recent improvements in air quality have come at a high cost as communities grapple with widespread lockdowns and shelter-in-place orders as a result of the spread of COVID-19," NASA said.

The data NASA released should be used with caution, Accuweather said. One reason is that NO2 levels recorded by satellites aren't necessarily the exact same as levels found at the ground level.

NASA also said in its report that further analysis is needed to determine the true amount that nitrogen dioxide levels have changed and whether it is associated with changes in pollutant emissions – or is just a natural variation in weather.

“We all have this hunch about how air pollution has changed, the longer this goes on, the more we will see,” NASA air pollution specialist Ryan Stauffer told The Washington Post last week.