While the coronavirus pandemic has kept Washingtonians close to home for months, it’s been decades since they’ve breathed fresher outdoor air.

The Washington region has yet to experience a day with unhealthful air quality in 2020, the latest on record that we’ve achieved this in a calendar year. Favorable weather, and lower emissions, have led to the cleanest air since at least 1980.

The reduced emissions are due to long-term pollution-control policies and the coronavirus pandemic, which has curtailed economies and taken thousands of polluting vehicles off the roads.

So far in 2020, the air quality index for metropolitan Washington has held in the good (green) range on all but seven days when it was moderate (yellow). There have been no code orange or red days for air pollution.

People can tell the difference. Capital Weather Gang Twitter followers, when asked whether they had noticed air quality improvements, responded affirmatively.

“My asthma is usually so terrible that I have to go to the doctor for new medication,” tweeted @JaySlacks. “I’ve only used my inhaler once this year.”

Another Twitter respondent, @sherrysherry1, replied poetically:

“Walks are easier

The sky looks clearer overhead

No hazy sky on the horizon

Stars are brighter, and able to see more stars at night”

This year’s first code orange or red day, assuming it happens, will occur more than two weeks after the previous latest first code orange day on record: June 1 in 2019.

Ryan Stauffer, an air pollution specialist at NASA who has been closely tracking this year’s air quality in the District and comparing it to previous years, says weather is the biggest factor in 2020′s lack of pollution.

Wet and windy weather helps wash away and disperse pollutants, and tends to improve air quality. In contrast, dry, stagnant, hot and sunny weather makes air quality worse. Since March, the weather has generally been wet and breezy, with relatively few hot days.

The second half of March through April were abnormally wet, and April and May were cooler than normal. Between March 1 and April 30, Washington saw measurable rain on 32 days, tied for the most on record.

“It seems like the biggest impact this year is definitely the weather,” Stauffer said in an interview. “We had so many days with rain, the pollution never had a chance to build up.”

Only three days this year so far have eclipsed 90 degrees, compared with a historical average of five, and it’s those days which are most likely to result in compromised air quality, especially when the air is stagnant.

“I would expect at some point this summer, we will get a code orange,” said Stauffer. “This is really rare territory at this point.”

It’s possible that days conducive to unhealthful air quality could arise between late next weekend and next week, when a steamier weather pattern takes hold, with predicted highs close to 90 for several straight days.

Though the weather has limited flare-ups in pollution this year, Stauffer attributed the reductions observed in recent decades to national, state, and local government regulations as a key factor.

As a measure of how much progress has been made, consider that in the 1980s, the first code orange day would typically occur in late March to early April, whereas that now happens in early May. In just the past decade, there’s been a substantial drop in the combined number of code orange and red days each year. Between 2000 and 2012, 40 to 70 code orange and red days per year were common. But no more than 14 have occurred annually since 2013, with none so far this year.

Regarding this year’s relatively clean air, the coronavirus pandemic and the lower emissions because of reduced traffic and economic activity are likely to have had an effect, but the magnitude is uncertain.

“I do think it’s contributing in some way, but it’s very difficult to quantify,” Stauffer said.

A briefing from the Metropolitan Washington Council of Governments provided to The Washington Post shows about a 40 percent reduction in vehicle traffic since late March, with a peak reduction over 50 percent in early April during the stay-at-home period. The briefing said pollution from non-road sources, such as construction, industry and commerce, also fell while electricity consumption declined.

The pollutants “NO2 [nitrogen dioxide], ozone and PM2.5 levels generally seemed lower after COVID 19 related restrictions were implemented in the Washington region,” the briefing stated.

The drop in air pollution is consistent with declines seen in other areas during the coronavirus pandemic. A study published last week showed a 48 percent decline in pollution from nitrogen dioxide in China during the lockdown period after that country’s outbreak of covid-19.

But with the Washington region reopening and the two hottest months of the year to come, air quality could deteriorate. And even current pollution levels, while the lowest in decades, can be improved.

“[T]he health effects of ozone pollution don’t stop below Code Orange, which means there’s still work to be done even if there are zero/few Code Orange days this year,” tweeted Dan Goldberg, a pollution researcher at George Washington University.