Ro membranes remove between 90 and 98% of impurities depending on the membranes specification, its the percentage they dont remove that is the problem.
In the third World they dont have to worry about removing pesticides and detergents etc. as we do. The villans are pathogenic microrganisms which kill millions of people every year!!!!
When you think that more kids die every hour from drinking local water than people that were killed in the twin towers it puts it into perspective.