Here’s a wake-up call for car fans who are inexplicably unconcerned about environmental issues: yes, this stuff can interrupt our precious race time, too. Low clouds pressed Shanghai’s smog even closer to the earth, per , stopping the first two Formula One Chinese Grand Prix practice sessions.

The sessions weren’t directly cancelled by smog per se, but for a completely different safety issue: poor visibility.

Fortunately, the days of nervously watching an ambulance speed away on surface streets after a driver is seriously injured are over. Nowadays, F1 drivers are airlifted by helicopter when things go seriously wrong, bypassing traffic and getting them to the emergency room smoother and faster. Unfortunately, when that helicopter can’t fly, F1 track sessions must be halted—which happened several times already during this weekend’s Chinese Grand Prix.

Even grassroots racing series have the same requirement. If there is no emergency vehicle on call and ready to go, most groups simply won’t run. My most recent race with the Lucky Dog Racing League—a budget-oriented enduro series that’s about as far from F1 as it gets—had to be delayed for an hour when the ambulance was late.

Helicopters are F1's emergency vehicle of choice, and given the more extreme speeds they travel at in open-cockpit cars, I don’t blame the series at all for saying all of the nope to running with no emergency helicopter.

Free Practice 1 was interrupted twice, with only ten cars able to set an installation lap before the first red flag was called for cloudy, smoggy conditions that would prevent the track’s emergency helicopter from being able to land at a local Shanghai hospital, per .

The slippery session—which was a bit too damp for cars attempting it on intermediate tires instead of full, deeper-grooved wet-weather tires—was halted for 40 minutes before restarting, with four more cars able to get out on track. When the damp weather caused the local airport to close, a second red flag was called that ultimately ended the session for good.

Red Bull’s Max Verstappen ran the fastest time of the interrupted practice session, with a . No driver was able to get more than eight laps in during the two shortened bits of FP1.

The low, smoggy, mist clouds didn’t disperse for Free Practice 2, which was when they couldn’t find any windows where the medical helicopter would be safe to fly into the hospital downtown.

Fortunately, Autoweek notes that weather conditions are expected to improve for Saturday, which should afford teams at least one full practice session in the new-for-2017 cars on Shanghai International Circuit before qualifying tomorrow. However, Sunday’s forecast is also for more rain, which could potentially postpone the race until the organizers are able to find an acceptable means for medical evacuations.