Nice project, could have saved a bit of time and used our sensor evaluation kit (AAS-AQS-UNO-RH-CO2), does dust, CO2, Hum & Temp, albeit doesn't do the FTP and web publishing you've achieved on your project.
Dust measurement in principle is easy, in practice quite difficult. The main reason behind this is the methodology used to measure the dust, mie light scattering; as dust particle refract light differently dependent upon their respective make up, type, and moisture content. The effect of humidity largely effects organic, rather than mineral based dust. And just to add to problems the reference sensors dry the dust. Add to the fact no two manufacturers use the same algorithms to translate the pulse counts, and you begin to see why dust sensors from diferent manufacturers rarely agree!
CO2 sensors should use Non-dispersive infra red (NDIR) principles, the VOC based values are estimates based on the VOC level, and commercial modules are more than good enough to measure residential variances.
Most ASIC based sensors self heat, we calibrate ours based on use type and voltage to overcome the offset, however we can't compensate for pick up within an enclosure. There is a reason most humidity sensors are thermally isolated from main PCBs! 1°C difference = 2.5~3% of humidity offset.
In all such projects we must remember that although the sensor looks like a chip, it isn't, and it has physical needs not just electronic ones, we come across the phenomena regularly in the professional world too where developers forget!