I used to work for a water treatment company in qatar and as far as i know Qatar has one of the world's best water purification plant and this water meets the standard as per indicated by WHO and the disolved solids in water which are recomended it has all that and if you read carefully on the botteled water you will find on that something written as TDS which means total dissolved solids. In tap water here TDS is between 130 to 180 depends on areas which is good for drinking but i totally agree with pwb78 that water is stored in tanks which are mostly on roof so it is not very safe for drinking.So the best thing is to buy bottled water for drinking and cooking purpose and yes you can use tap water for brushing teeths it has no harm in that.