A lot depends upon the nature of the impurities with which you must contend. A lot of my experience has been in relatively high mountains or lightly inhabited areas where industrial pollutants are trivial. Other locations may be different. It also depends upon whether you are talking short term exposure or long term use.

About twenty years ago, I spent a month in rural China on a caving expedition. We were way off the tourist circuit and depended exclusively on local resources and cooks. Our drinking water was treated exclusively by boiling by those cooks - standard practice for preparation of potable water at the time. Twelve of us lived locally for a month. Absolutely no cases of "Mao's revenge," which surprised me greatly. That is one reason why I am such a fan of boiling as a treatment method.