The chemicals enter your bloodstream when you smoke it and ultimately get metabolized by your liver anyway.

If you are concerned about pesticides, grow your own, or find a source who can guarantee that you are getting organic buds, just as you would for the food you eat.

Luckily, the pesticides commonly used on cannabis are things like pyrethrin, neem, and sulfur- the first two being very VERY safe plant-derived substances, and the latter being used as an anti-fungal in many types of greenhouses, not simply ones that produce cannabis.

Good question though.