TEOM stands for tapered element oscillating microbalance. This instrument measures particle concentrations in air continuously. Sampled air is drawn through a filter which collects particles. The filter sits on top of a hollow tapered element which vibrates at a known frequency (oscillates). The frequency of oscillation changes by an amount proportional to the mass of particles deposited on the filter. Because the sample air is heated to a high temperature the semi-volatile fraction of the particle mass can be destroyed. For this reason TEOMs can underestimate PM levels. Fitting a TEOM with a FDMS (filter dynamics measuring system) can overcome this problem.