1/ Since I'm seeing a lot of misconceptions about air sensing, here's a thread to help clear up some of those misconceptions. For one, there's no consumer sensor or array of sensors out there that provide an accurate picture of air quality.
2/ In fact there's not great real time systems at any price level for sensing air quality. To get a true picture, you need lab equipment, and sometimes you have to wait to complete a whole lab method.
3/ Let's start with particles, since I've seen a lot of discussions on those lately. The most accurate particle counts come from gravimetric devices. Most consumer sensors though, work on a much simpler principle.
4/ This is the inside of a Temtop particle counter. It's reasonably made, with a small fan to draw air in to try to make the sampling more accurate. 
5/ Once the air is inside, they measure the reflectance/deflection of a laser to count how many particles are coming through. Presumably the particles should deflect or reflect the laser so they can be counted.
6/ There are some problems with that though! For one, humidity in the air can also refract light. So this can create a 'mirage' effect, sensing particles when they aren't there.
7/ Overall, when studied, these kinds of particle counters at best can get within 2X of the accurate answer. They can be useful for trends (is it going down or up?) but not generally useful for absolute values.
8/ VOC & gas sensors are typically much worse. Most consumer gas sensors are electrochemical gas sensors. They function by running a redox reaction at an electrode and measuring the resulting current. There are many problems with this approach.
9/ This could be fine if you could control exactly what gas your electrode is exposed to, and calibrate the current based on exposure to known quantities. But air is composed of a lot of things that can react in a lot of ways.
10/ My experience testing these sensors is that even the best of them can not get you to even directionally correct. That means you really can't trust the reading going up or down outside controlled scenarios.
11/ The gold standard for testing gasses is GC/MS. This is gas chromatography and mass spectrometry used concurrently. I'm not going to go into details on how it works but you can read about it here: simple.wikipedia.org/wiki/Gas_chrom…
12/ What we use in our lab is both GC-MS, HPLC (High Pressure Liquid Chromatography) and a realtime Photo-Ionization Detector for quick and dirty results. The PID probe isn't 100% accurate, but it provides reasonable and realtime results.
13/ The GC-MS or HPLC require sampling, extraction, and then running the samples. So it's definitely not realtime.
14/ Trying to measure bioaerosols (for ex. Mold, Bacteria, Viruses) is even harder. There are some real-time approaches around phosphorescence, and some commercially available products there, but most of the time you have to culture them, incubate, and wait.
15/ Most of this info is quick to digest based on what we've learned to educate based on the misconceptions I've been reading here on twitter, but we'll have a more detailed blog post explaining with more nuance out on our site in the future for those interested!
16/ Just a bit of outside reinforcement for y'all: eta.lbl.gov/news/article/h…
Some important quotes: "From a health perspective, where you really need high confidence in a monitor's numbers to assess an environment, we're not quite there yet."
However, all of the monitors faltered for particles with diameters smaller than 0.3 micrometers, which have been associated with a variety of adverse respiratory and cardiovascular health effects.
These areas are super important to us at Molekule, because we care about how we can affect and improve health first and foremost. It's why we didn't just to extensive air quality testing, but also focused measuring health impacts: journals.sagepub.com/doi/full/10.11…
