If you're worried about pollutants or chemical attacks, you could surround your home with sophisticated (and expensive) detectors. In a few years, you might be able to do the job with specially engineered plants that instantly change color in the presence of a harmful substance.
A research team at Colorado State has taken the inherent abilities of plants to detect chemicals and altered them to detect specific substances. Once the substance is detected, a process is triggered that suppresses chlorophyll, turning parts of the plant white. A bunch of suddenly pale plants could tip you off to the meth lab next door, the terrorist cell making bombs, or the methyl isocyanate leaking from the Union Carbide plant down the street.
Currently, the most most efficient way to detect a range of harmful chemicals is with dogs who can sniff them out. The dogs require extensive training, of course, and they represent a centralized, sporadic method of detection. When the dogs go away or take a nap, you can't detect things. So-called "sentinel plants" could be placed liberally in strategic locations, requiring minimal upkeep and creating a decentralized network of chemical detectors.
The research team, which was funded in part by the Department of Homeland Security and DARPA, used a computer to modify a receptor protein to recognize a specific substance. The protein is introduced into genetically modified plants that include important bits of bacterial DNA (bacteria are very good at detecting chemicals, but not good at consistently growing on your front lawn). The receptor is then linked to a plant's natural signaling system, known as histidine kinase (HK) mediated signaling. This is turn activates something called the degreening circuit, which causes the plant to turn white.
Researchers are confident they can adapt the system to detect a wide range of chemicals. They also want to reduce the reaction time, which takes several hours right now. Other improvements might include altering the color change so that it's only detectable via infrared, so terrorists don't immediately know when they've been spotted.
Some day you might keep such a plant in your kitchen. It will turn red if it detects radon, blue for carbon monoxide, and it will turn white in the presence of alien gokoron spores, which they've been known to use since they first invaded in 2019.
Sources: EurekAlert! Sentries in the garden shed.