workingApparently, each person can shed as much as 1g of skin per day. This dead skin is colonised by saprophytic fungi (whatever they are), and then by dust mites (Dermatophagoides, which means skin-eater. Yum). Dust mites are arthropods measuring 300 micrometers (or 0.3mm), and grow from egg to adult in 25 days. Here is the really disgusting bit: floor or mattress dust samples typically contain 100 to 1000 mites (dead or alive) per gram. Do the maths *shudder*.
Mite faeces are sticky pellets about 20 micrometers (0.02mm) in diameter, and constitute a large proportion of the allergen in house dust. The gastric enzymes produced by the mites become airborne and are eventually inhaled. The concentration of the two specific allergenic proteins necessary to produce an asthma attack in susceptible individuals is as little as 5ng per m^3 of air (ng are nanograms, or 1 x 10^-9g)! Meep!
Significantly more relevant to my research, typical concentrations of the pollutant called "particulate matter" (basically, all solid and liquid material suspended in air) in urban environments are 10 to 50 micrograms per m^3 of air. A "pollution episode" (a time of extremely high pollution, when people with respiratory disease are warned to take extra precautions) would be defined as a concentration of around 100 micrograms PM per m^3 of air. I myself know from looking at air quality information that I notice changes in PM concentration in the order of 10 micrograms per m^3 air: it's enough to make the difference between my preventer inhaler working or not.
As you're probably aware, in the middle of the 20th Century, London suffered a number of extreme smog events. Apparently, on 3 successive days in 1962, the peak particle loading in London was measured as 7000 micrograms per m^3. (No, I didn't put an extra 0 in by mistake). Amazingly, only 700 people died as a result of this: there is absolutely no question that if I had been around then, I would have ceased to be around. I find it absolutely unbelievable that the air could have been that polluted. And we think we've got it bad now...
