Ozone, a form of oxygen with three atoms instead of the normal two, is beneficial to all living organisms, shielding and filtering the Earth (and its organisms) from excessive, damaging, and life-threatening ultraviolet (UV) radiation generated by the Sun. But the “dark side” to ozone is that near ground level, ozone is a major pollutant that helps form photo-chemical smog and acid rain. In fact, ozone in this form is highly toxic—less than one part per million of this blue-tinged gas is poisonous to humans. For this reason, especially in big cities, the weather bureau often issues an ozone alert for people with upper-respiratory health problems (or for everyone if the concentration of ozone is high enough)—in particular, if an air mass that can hold the ozone in the lower atmosphere is stagnant.