Is powder used to clean HDB walkways safe?

In their reply on the recent mass fish deaths, the PUB and National Environment Agency (NEA) advised that used water "should not be discharged into the drains but into sewers" and that "drains are meant to convey rainwater to our reservoirs or the sea" ("Mass fish deaths: PUB, NEA reply"; last Wednesday).

I have noticed on some occasions that some kind of white powder that smells like bleach has been left on walkways in Housing Board estates, probably by cleaners for washing these areas.

I do not know if the powder is bleach or caustic soda, and wonder if it could be detrimental to our health when inhaled regularly.

Will this powder or chemical used to clean our HDB walkways go into our drains and, thus, into our reservoirs?

Joan Chng (Ms)