A detector of harmful chemicals installed at the intake of a central ventilation system that automatically pauses external air intake until the threat is gone.
Some harmful airborne substances (smoke, chemical fumes, etc) are unlikely to get filtered by a standard ventilation unit. If they reach the ventilation intake the entire inside of the building will get contaminated.
This could be a universal module that can be added to any ventilation system. The module would monitor the air for any known/supported substances that are likely to pass the filter. For example smoke is the most obvious/expected culprit in residential areas. The module could be upgraded with additional cartridges that detect other likely molecules (ozone, formaldehyde, paint thinner fumes, and whatever else is likely in the area).
The module would connect to the ventilation controller. If the function is available it would stop the external intake and switch the system to internal air circulation. If there is no such option (in small residential units) it would temporarily cut the power of the ventilation system and start beeping to notify people about the threat.