Health officials are warning of potentially unhealthy air pollution along Colorado's Front Range this weekend due to high levels of ozone.
The Colorado Department of Health and Human Services issued an ozone alert that was to remain in effect until at least 4 p.m. Sunday.
The agency says active children and adults and people with lung diseases such as asthma should reduce prolonged or heavy outdoor exertion.
The affected area includes southern and western portions of the Denver area, northward to Boulder and Fort Collins.
Ground-level ozone is created when vehicle exhaust, paints, cleaning agents and other chemicals react with sunlight. It typically gets worse in the summer, and can cause serious respiratory problems and trigger asthma attacks.