S h o r t - t e r m P o l l u t i o n Episodes a n d t h e N e e d f o r a S m o g A l e r t S y s t e m There are occasions, short-term episodes (smogs) lasting a few hours or days, w h e n air p o l l u t i o n concentrat ions reach health-threatening or dangerous levels wel l above air quality standards. Smogs occur when urban emissions are relatively high and stagnant meteorological condit ions fail to dilute and disperse the pollutants sufficiently. Concern over the adverse heal th effects of smogs has led to many c i ty and nat ional author i t ies i n t r o d u c i n g schemes to w a r n of, as we l l as forecast, the occurrence of smogs. The authori t ies can then advise the c o m m u n i t y of the potent ia l health threats of the poor air quality and the actions the publ ic can take to min imise their exposure to h igh po l lu t ion levels . The authori t ies can encourage voluntary actions or require mandatory short-term measures to reduce pollutant emissions during the smog.