You can’t see it, smell it, or taste it. This invisible pollutant even accumulates the most on hot and sunny blue-sky days, when the Colorado air appears clear. Yet, ground-level ozone is the Front Range’s most pressing air quality problem, impacting our health and environment each summer. Ozone is simply three oxygen atoms stuck together. High up in the atmosphere, ozone serves an important purpose: it protects us from the sun’s ultraviolet radiation.