A drug is a chemical and a chemical is a drug.  Just as most drugs are
not a problem because the risks of formula feeding are greater than the
risk of breastfeeding with tiny bits of drug or chemical in the milk,
usually this type of exposure does not negate the benefits of
breastfeeding or the risks of formula feeding.  It is possible to
measure quantities in milk, if the mother so wishes.  Most chemicals
that are inhaled are quite quickly exhaled again, with only small
quantities entering the body.

Jack Newman, MD, FRCPC