What led to MLB’s decision to play Tuesday’s Chicago Cubs game at Wrigley Field in ‘very unhealthy’ air quality conditions?

Baseball fans have been inundated by new stats over the years, from WAR to wRC+ and exit velocity to barrels.

Welcome to the club, AQI.

Air Quality Index, the latest addition to the baseball glossary, has been picking up speed since early June, when the New York Yankees, Philadelphia Phillies and Washington Nationals had games postponed due to poor air quality from the wildfires in Canada.

The Chicago Cubs were the latest team to deal with air quality issues Tuesday when the AQI in Chicago reached a level deemed “very unhealthy.” MLB had several discussions with the Players Association before gametime, but ultimately opted not to postpone it, which resulted in Philadelphia’s 5-1 win in a smoky haze.

Ian Happ, the Cubs player representative, said afterward the union was consulted on the decision, adding “the league felt the levels were playable” and “there wasn’t going to be any harm” if they played.

“You smell it, you taste it, but you just go out and play baseball and forget about it when you go out there,” he said.

How did MLB deem it was safe to play?

MLB, which has the authority to postpone games due to air quality, made its decisions after speaking with multiple medical experts, weather forecasters, the teams and the medical staff of both clubs. It was nothing new, unfortunately. Wildfires on the West Coast in September 2020 prompted MLB to postpone two games in Seattle between the Mariners and Oakland A’s, moving them to San Francisco’s Oracle Park.

An AQI above 300 is deemed hazardous by the EPA, while a number between 201-300 is considered “very unhealthy.”

The AQI at Wrigley on Tuesday night was in the 250 range.

Happ said when the Yankees-White Sox game was canceled on June 7 “it was crazy, (the AQI) was 300-plus” when players arrived at the stadium. After the cancellation led to a doubleheader the next day, Sox third baseman Yoán Moncada, who had a difficult recovery from COVID-19 in 2020, wore a mask over his face during the rescheduled games, while closer Liam Hendriks, who recently ended chemotherapy treatment for cancer, was held out for precautionary reasons.

Hendriks told ESPN-1000′s “Waddle and Silvy” show afterward he felt like he had a head cold and nausea from the air pollution in New York and was “struggling” to feel OK.

“It wasn’t an ideal situation,” Hendriks said, adding he told manager Pedro Grifol he could pitch but appreciated being off.

MLB doesn’t have a Mendoza line, so to speak, for AQI to decide what level is acceptable to play in. Happ said it’s an “arbitrary” number and did not know how MLB made its decision.

“There’s no line, like (saying) ‘Well, 250 is bad, 300 is worse, but 225 is good,’” Happ said. “I talked to (Kyle) Schwarber a little bit this morning, but everybody was just trying to get a feel for what was going to happen.”

According to league sources, the AQI is just one factor used by MLB in its decision-making and there’s no set number that would automatically trigger a postponement.

Neither team likely wanted to play a doubleheader Wednesday. The Cubs just returned from a long road trip to London and the Phillies are on a 15-5 streak and didn’t want to lose momentum. The Cubs also sold 37,071 tickets and a small minority of fans wore masks on Tuesday night.

Cubs starter Jameson Taillon said his legs were “tired” from the long flight home, but the smoky conditions didn’t affect him in a subpar, five-inning outing.

“The smoke was definitely weird,” he said. “But once you’re out there for a little bit, I feel like you get acclimated to the smell and the density in the air.”

Happ said players probably won’t know for a day or two “if there are any effects” from playing in the unhealthy environment. He didn’t argue with MLB’s decision, pointing to the unusual circumstances involving air quality issues teams usually haven’t had to worry about.

“We’re all trying to figure it out together,” Happ said.

The AQI in Wrigleyville on Wednesday morning was at 209 and another air quality alert was issued by the National Weather Service. Manager David Ross said there were no discussions Wednesday about postponing the game./ The Cubs and Phillies are scheduled to play at 7:10 p.m.

Masks are optional.