If you reside near any industrial works, and you don't have a rainwater storage tank and intend collecting in buckets or the like, let it rain for a while before collecting it. Impurities or pollutants in the atmosphere from those works will by then be mostly filtered out by that rain resulting in better quality rainwater - as good as the quality of rainwater can be that is, if you get my drift.
In fact, collecting it in buckets may be better than a storage tank, cos all those impurities and/or pollutants which may be present in the atmosphere will still end up in the storage tank in the form of run off from the roof, besides organic matter from leaves etc etc. One can only do the best one can. From memory mine came in at around 45ppm straight from the tank.