Last week, many Minnesotans in the Twin Cities area were surprised to be advised not to engage in strenuous outdoor activities because of “bad air.” The Air Quality Index, an air pollution scale, peaked last week in Minneapolis at 155 – into the “unhealthy for all” range, something that has not occurred for approximately 25 years.

The metro area spent most of last week in the “unhealthy for all” or “unhealthy for sensitive groups” categories. To reduce emissions, the Minnesota Pollution Control Agency discouraged unnecessarily idling cars and burning fires in fireplaces. According to the agency, the particulate matter trapped in the air comes from combustion – car exhaust, industrial sources, fireplaces and the like.

But why was this the first such problem in 25 years? The weather – specifically, a broad, surface high-pressure system moving across the Midwest – caused a temperature inversion, reduced atmospheric mixing and stagnated the air. Particles that would normally be carried off and dispersed stuck around, and those pollutants then contributed to the formation of secondary particles via chemical reactions of sulfates and nitrates with the moisture in the air. The snow cover further intensified those reactions by providing ample moisture near the ground, and all of this made the air unsafe.

Last week’s phenomenon is not something Minnesotans deal with regularly. More-humid and polluted areas, such as Los Angeles, experience such warnings much more frequently. Nor does it imply Minnesota is now producing more air pollution than ever. But the warnings should open our eyes to the dangers of air pollution in general, now that we have seen the amount of pollution that can accumulate – to the point of being dangerous – in just a few days of stagnant air.

These warnings are all the more reason to do everything possible to reduce air pollution. Carpooling and public transportation help reduce fuel emissions. Better yet, take advantage of the beautiful weather to walk or bike, if those are options for you to get to class or work. And finally, support programs and legislation that help reduce air pollution through public transportation and emissions standards. Hopefully then, Minnesotans can keep breathing fresh air.