Using density functional theory, we have studied the trapping of formaldehyde on hydrogenated boron nitride nanosheets. On the nitrogen terminated side, the formaldehyde molecule is physisorbed at a distance of similar to 3.1 angstrom on top of a N atom. A calculation of the adsorption energy, as function of vertical separation between the molecule and the substrate, shows that closer to the surface, there is a strong repulsion due to the electronegativity of the N and O atoms. On the other hand, the trapping of formaldehyde by the substrate on the boron terminated surface is very favorable. The reaction of a single molecule is described by calculating the minimum energy pathway. It begins with the formaldehyde molecule and the boron nitride substrate far away from each other. In the following state of the reaction, the molecule attaches to the boron side of the substrate, gaining a large amount of energy. Through this interaction, the double bond of the oxygen atom breaks down, turning the molecule into a highly reactive carbon centered radical. In the final state of the reaction, a neighbor hydrogen atom is abstracted. In this state, there is an additional energy gain of 0.06 eV. Once the stable molecule is formed by the abstraction of the hydrogen atom, a new dangling bond is created at a neighbor boron atom, which it may serve as a new site for the following molecule to attach. In this way, a chain reaction is possible. This self-propagating reaction is more viable for hydrogenated boron nitride than for graphane, opening an avenue to use hydrogenated h-BN in the indoor air pollution control. When there are two or more neighboring hydrogen vacancies in the substrate, the formaldehyde molecule prefers to attach forming O-B and C-B bonds with the substrate.