If you have someone at home that has allergies or other health issues, it's nice to be able to track. There are a host of startups and home devices that track this and let you see them in the cloud or on your phone:

• PM2.5. These are the small particles that get past your defenses. Ideally, you should have filtered air, 2.5 micron particulate
• PM10. If there are molds or other things in the air, then you should know about them. This standard for 10 micron particulate.
• VOC. Volatile Organic Compounds such a Formaldehyde are also detected.
• CO. Carbon monoxide. The classic leaking stove problem.
• CO2. Carbon dioxide.
• Humidity. High humidity plus high PM2.5 count could mean mold is in the house.

There are not many good quantitative reviews, but CleanTechnica did a nice set comparing the major brands, but here's the summary:

1. Foobot. This has the best in terms of software and and also sensors and calibration. It is nice because it has PM2.5 and VOC but also wifi access and IFTTT integration so you can get alerts and control other things as well. $199 on Amazon There are some others that are just coming like: • uHoo.$270 and it has one Ozone as well and mobile apps.
• Airvisual. This is mainly a PM2 and CO2 monitor and has a fantastic crowdsourcing system for monitoring outdoor PM2. Also \$270 Direct. But it doesn't have VOC.