The Department of Natural Resources says air monitors in east-central Iowa recorded pollution today above the minimum level set by the E.P.A. The D.N.R.’s Eddie Elkin says the minimum is 35-micrograms. He says the level was 50 in Iowa City and Muscatine, with an average of 45 in Cedar Rapids and 49 in Waterloo.

The number measures what are called “fine particles” in the air. Elkin says the weather is holding the particles in the air and causing the high readings. Elkin says there’s a temperature inversion and a stagnant air mass that doesn’t allow the particles to disperse. He says there’s a daily cycle from the nighttime to the morning, where pollution levels rise and then drop as the temperature warms up.

The pollution levels rise again when the temperature drops. Elkin says the fine particles are emitted by vehicle traffic and other sources such as manufacturing plants. He says the air can cause problems for people with illnesses such as heart or respiratory disease.