I saw Arrow selling this Amphenol/Telaire SM-PWM-01C SMART Dust Sensor for $9 domestically and said WELL OK THEN and picked one up. It detects dust particle conceptration by using an IR diode and a photo-sensor to detect reflected light. The sensor is supposed to distingish between small particulates like ciagrette smoke vs. large house dust based on the pulse pattern. The manufacturer once (perhaps still) sold an environmental development kit that includes this sensor along with an Uno.
Operating voltage is 5V, supply voltage is 4.75 to 5.25 and it consumes 80 to 100 mA. An average reading delay of 30 seconds is needed for stable readings. The sensor needs to be installed vertically so air can flow from the bottom opening (resistor) to the top.
1 > GND
2 > P2 Signal Out (large particulates, 3-10 �m)
3 > +5V
4 > P1 Signal Out (small particulates, 1-2 �m)
5 > N/A
Neils Roberts has provided dust sensor code for interrupt reading and a count method. The code is well documented, but note it samples every 5 seconds rather than the recommended 30.
dantudose provides a SM-PWM-01A library, which is for an earlier model. My readings with this library fluxuated considerably.