The danger from the first of these, namely atmospheric pollution, seem slight. The probability of finding truly-airborne ingestible pathogenic viruses or bacteria seems low and of finding larger airborne pathogens negligible. Interest therefore focuses mainly upon contamination of roofs and the performance of water stores in reducing or increasing pathogens. Roofs and gutters are made of a variety of materials. Contamination of water might arise from the roofing material itself or from substances that have accumulated on a roof or in a gutter.
Metal roofs are normally of treated steel or less commonly of aluminium.
Aluminium is very inert unless in contact with very acid water. Corrugated steel roofing employs mild steel protected by hot-dip or electrolytic galvanising or by painting, since stainless steel is too expensive to use. More seriously although no one can now afford lead sheeting on roofs, localised lead flashing is still used at joints. One study in Malaysia Yaziz 1989 reported lead levels of up to 3.5 times WHO limits in roof runoff but this is not a general finding and seems to have arisen from lead in dust deposition rather than the roofing material since it reduced rapidly with storm duration. The danger from inhaled asbestos dust is however sufficiently high that working with asbestos sheeting, for example sawing it, without special protection is now generally banned in industrial countries.