Morgan Stanley analysts on Wednesday said they are tracking air quality reports in Chinese cities that suggests an increase in industrial activity tied to iPhone 12 production.

Two new iPhone 12 models went on sale last week and two more will go on sale next month. This is a few weeks later than Apple’s traditional cadence, which aims to release new iPhones in September, and the delay is most likely due to disruptions caused by the Covid-19 pandemic. Apple investors are looking for clues as to how the production of the finished devices is going in order to better project sales in the coming quarters.

For Morgan Stanley, air quality is one such clue.

“Using air quality data from a non-profit platform that collects and publishes air quality data in China, we track nitrogen dioxide levels (controlling for temperature, humidity, wind, etc.), which, according to the European Space Agency, is a first-level indicator of industrial activity, in four Chinese cities where Apple’s manufacturing partners have a large manufacturing presence,” the analysts wrote.

In Zhengzhou, China, a major production site for Apple’s manufacturing partners, the analysts suggest lower air quality shows a ramp in production.

“As of October 26th, air quality data from Zhengzhou, also known as ‘iPhone City,’ shows that industrial production levels have spiked higher recently, about a month later than historical seasonality, which we believe coincides with the ramp in iPhone 12 mass production,” Morgan Stanley analysts led by Katy Huberty wrote in a note this week.

In Shenzhen, Morgan Stanley believes industrial production ramped higher in early September, but dipped below historical seasonal levels. In Chengdu, the analysts found that industrial activity is ramping higher in recent days. In Chongqing, industrial production has ramped meaningfully higher in recent days after an uptick and pullback in September.

Apple reports its fourth fiscal quarter earnings on Thursday. However, the period Apple is reporting results for ends in September, so it will not have any iPhone 12 sales included.