Presents new empirical formulae for the prediction of the air velocity in front of rectangular (slot) openings. Infers the equations from an experimental study of a common type of local exhaust opening. The formulae describe the three-dimensional velocity field near a rectangular exhaust hood, providing the velocity gradients along suitable directions, parallel to the slot axis. Testing of the formulae showed a good fit to the experimental data. The authors emphasise the simplicity and user-friendliness of the method for predicting the air velocity in front of a suction opening and for correctly dimensioning a local exhaust ventilation system.