The present invention is a push-pull ventilation hood having a push device to obtain a push air flow and a pull device to exhaust contaminant flow with an exhaust opening and the two devices properly coordinated with each other, wherein, when the push flow flows through a contaminant source, a contaminated flow of the push flow is exhausted through the exhaust opening. The database management system provides access to a hierarchical tree representation of surfaces of geometric models. Compounds the invertebrate can detect include skatole and androstenone. A multiplication region is defined proximate to and separate from the absorption region. A needle is slidably disposed in the lumen of the guide.