There is NOTHING that will immunize an area from the approach of a drug-detection dog. However, there ARE certain chemicals that may interfere with the dog's perception of smell. One is cayenne pepper. Another is ammonia and camphor. A third is Citronella oil.
Cayenne pepper is too spicy and irritates a dog's nose, thus persuading him against trying harder to perform any decent detection. Ammonia is another strong chemical and is actually used alongside Camphor as a method of halting a pursuing police tracking dog because it is too powerful for the dog to continue tracking. Explanation: Tracking dogs track people through the miniscule secretions of ammonia the human body releases. Overloading a dog's senses with ammonia is roughly comparable to dropping a room full of people directly on top of a tracking dog without notice. Citronella oil is a common ingredient in many insect repellants and obediance collars. This smell is also incredibly unenjoyable for a pooch if held in concentration.
All in all, there is ZERO chance of completely removing the threat of a drug dog's powerful nose, but there are many actions when, combined together, can greatly reduce one's chances of being discovered by a dog. I don't condone relying on this to think you have the system beat because nothing can be completely fool-proof. Best air on the side of caution. However, it doesn't hurt to keep a small bit of ammonia or citronella (or ammonia AND citronella...use your imaginations
) in the bottom of a water bottle easily accessible in a vehicle in case the situation may call for it.
something i pinched off another site