I understand your concern, HouseOfWool, but it can be done safely. Some people collect rainwater for household use (showers, toilets, washer, etc.) and even to drink if a disinfection system is used. My grandparents old home in western Iowa was set up to collect rainwater off the house roof as the drinking water source in the late 1800s and early 1900s. Many rural homes in the Southwest use rainwater collection even today, rather than truck water out from town.
The usual procedure for a roof collection system is to let the rain that falls in the first 20 minutes or so just drain normally to the ground, then the rainwater is diverted to a storage tank. That lets the dust, shingle gravel, etc. wash off. It works pretty well, if it's set up right. A metal roof is probably the best, agreed, but an asphalt shingle roof works fine, as long as it gets washed off well before the rainwater is collected.
Pollution can be an issue. Particulate pollution (smoke, dust, etc.) can be removed with a decent filter. Non-particulate pollution (acid droplets, chemical vapor, etc.) can be a concern, since the chemicals are dissolved in the water. If you live in an urban area, it's worth investigating this issue. In rural areas, the concern is not so great, if only because contaminants in the air get diluted a lot.
PS: Judy and Paillo -- thank you. I appreciate the warm feedback!
