Clearly, the reliability of the results from the box model approach depends on a number of assumptions such as uniformity of dispersion. At any specific receptor site within the box, this assumption is typically violated, particularly when the averaging time is very small. The box model has also been applied to nonhighway modes. Cohn and McVoy (1982) cited an example of the FAA box model that can be used to assess CO emissions at airports. In the case of airports, the receptors are passenger loading areas (where emissions are from ground aircraft and service vehicles) and passenger pickup and drop-off areas (where emissions are from highway vehicles dropping or picking up passengers). Whereas the short-term maximum concentrations in such areas may be unbearable to persons (receptors) at such points, the overall average concentration throughout the entire airport box space may be too little to be of concern. The box model therefore may underestimate air pollution severity, particularly at localized but sensitive receptors.