I backed the Kickstarter for bluSensor last year, and have been happy with the device so far. The app needs a little work, but makes it easy to check current and historical air quality.

It has 4 sensors: temp, humidity, CO2 and VoC. Helped me discover a problem with my attached garage leaking fumes into the house.

https://www.kickstarter.com/profile/blusensor/created