Patently Apple:

Today the US Patent & Trademark Office published a patent application from Apple that relates to future iDevices being integrated with poisonous gas detectors. Carbon monoxide (or CO) is an odorless, tasteless, invisible gas that is sometimes called the silent killer because it poisons and kills many people each year, without them ever being aware of the danger. Having a miniature gas sensor built into an iPhone or Apple Watch will be able to notify a user that they could possibly be in a dangerous environment at home, at work or in the public.

Carbon monoxide detectors are usually mounted on the ceiling. And carbon monoxide is slightly lighter than air. Might Apple have plans for their own HomeKit-enabled, ceiling mount detectors?

The patent (and a related patent) does show the sensor in an iPhone and MacBook.