Unpleasant sulphur smells coming from your sink drain may be either sulphates or hydrogen sulphide gas in your water. Sulphates gives water a bitter taste and odour, and causes scale build-up on water pipes. Hydrogen sulphide, distinguished by a "rotten egg" odour, causes yellow or black stains on fixtures; it affects the taste and smell of coffee and food. Although hydrogen sulphide is flammable and poisonous, it is usually not a health risk at the concentrations found in household water. The World Health Organization Water Sanitation declares it unlikely that anyone could consume a harmful dose of hydrogen sulphide in drinking water. Sulphates and hydrogen sulphide are not always responsible for sink drain odour, however; the smell can also be caused by something as simple as decaying organic matter trapped in your drain.
Open a window to increase air circulation and pull some of the odour outside.
Utilise a plug-in air freshener in a refreshing citrus or pine scent to mask odour until you can solve the problem.
Check the drain cap for rotting food that could be stuck underneath. If you have a garbage disposal, run it briefly to get rid of any decaying food.
Pour 1/2 cup of white vinegar and 1/4 cup of baking soda down the drain, followed by a potful of hot water. Vinegar has antiseptic, antibacterial and odour-neutralising properties; baking soda's alkalinity helps it dissolve clogs. If you have grapefruit, lemon or orange peels available, put them down the drain and run the garbage disposal.
Turn off the garbage disposal and use a stiff wire brush dipped in baking soda or dish detergent to scrub underneath the rubber flaps to remove any decaying food that has collected there.
Use a commercial drain cleaner that contains sodium hypochlorite, such as Drain Pro, if the problem persists. Your sink drain may be blocked with decaying organic matter.
If odour still persists, call in a professional. Have the water heater's magnesium corrosion control rod checked; sometimes this rod can malfunction and convert naturally-occurring sulphates to hydrogen sulphide. A plumber can also check for incorrectly installed plumbing, missing or leaking pipes, a non-existent S-bend, and the presence of spores and mould. He will also check the pipes to rule out the possibility that the odour is due to leakage of sewer gas, which in rare cases could cause methane gas exposure and hydrogen sulphide poisoning.
In rare cases, hydrogen sulphide odour results from sewage pollution, which may contain disease-producing contaminants. A persistent sulphur smell coming from any drain should be checked out by a professional. Read drain cleaner label, and follow directions precisely.