The Iowa Department of Natural Resources says ditry air was detected in Iowa again Tuesday. The D-N-R’s Brian Button says levels were high across much of eastern Iowa on Tuesday and even higher yesterday in Scott and Clinton counties. He blames a weather phenomenon called a “thermal inversion” That’s when colder air is held near the ground by a layer of warmer air above, and it acts like a lid trapping particulates and emissions from tailpipes and industries. Button says the hazy sky these days is because of microscopic particles that scatter light in the atmosphere, and he says the particles and soot sent out in things like vehicle emissions are very small and penetrate deep into your lungs. While we think of hot muggy weather as a time to encounter air-quality alerts, Button says they can happen any time of year.On such days he says anybody with heart or lung problems should avoid prolonged outdoor exertion. Actually, he says even healthy people should avoid too much outdoor running, cycling or other vigorous outdoor activity if it remains bad. The DNR air-quality specialist can’t say if today will bring another alert but they’re monitoring the quality in eastern counties where it’s been bad, and will send word as soon as the situation’s determined.