After looking over many options for an air-quality sensor, I settled on the SDS011. It costs about $25, and can be ordered on many Chinese retail sites, such as AliExpress.
The SDS011 is well reviewed, and “developed by inovafit, a spin-off from the university of Jinan”[sic]. It reports the concentration of ultra-fine (2.5 micron) and fine (10 micron) airborne pollution in μg /m3, which are standard measures called PM2.5 and PM10, respectively.
There are several other sensors, but the SDS011 had the benefit of being well reviewed and capable of being connected over USB. I found that soldering joints and using breadboards were very difficult and unreliable.
The sensor has problems, though. The documentation is sparse, and it does not come with a program to make it function and record the data. These have to be written (or downloaded). The specifications are also written in poor English.
Finally, I found it hard to believe that a $25 sensor would do a good job—that it would be accurate, reliable, and consistent with other sensors. I was glad to be mistaken about these concerns.
2. The World Air Quality Index. The SDS011 Air Quality Sensor experiment. aqicn.org Available at: http://aqicn.org/sensor/sds011/. (Accessed: 30th May 2018)