Too many days with poor air quality put Denver at No. 5 on a list of big U.S. counties where man-made environmental hazards are prevalent.

RealtyTrac ranked 578 U.S. counties with population of at least 100,000 based on percentage of bad air quality days, Superfund and brownfield sites, polluters and former drug labs per square mile.

Only St. Louis, Philadelphia, Baltimore and Hoboken, N.J., exposed residents to higher levels of risk, according to the assessment.

But at least in the short term, exposure to pollution and other hazards hasn’t harmed the real estate market, according to the report, which also looked a median home values and appreciation, unemployment rates and median household income.

“Somewhat surprisingly, short-term home price appreciation over the past year and five years is stronger in the 50 housing markets with the highest prevalence of man-made hazards,” RealtyTrac vice president Daren Blomquist said in a release.

Over a 10-year period, the 50 markets with the lowest prevalence of man-made hazard performed better, he said. The lowest risk areas were Bend, Ore., Duluth, Minn., Saint Lawrence County, N.Y., Skagit County, Wash., and Seattle.

Based on reporting by the U.S. Environmental Protection Agency, air quality was poor in Denver on about 10.8 percent of days per year, compared with an average of 5.43 percent nationwide, RealtyTrac said.

Air quality was worse in adjacent counties, with Arapahoe and Adams reporting bad air 11.97 percent of days per year, and Douglas with 11.

But the suburban counties ranked far down the list for prevalence of man-made hazards: Arapahoe, 32nd; Adams, 38th; and Jefferson, 45th.

The Denver Metro/North Front Range region fell out of compliance with federal standards for ozone pollution in 2007 and 2008, according to Meg Alderton, spokeswoman for the Regional Air Quality Council.

She said the EPA has given the region until 2015 to meet the 2008 federal standard for ground-level ozone pollution.

“”We are designated currently as a marginal non-attainment” for the National Ambient Air Quality Standards for ground-level ozone.

Still, air quality in general has improved in the nine-county region covered by the air quality council since 1980.

“We had been out of compliance in the 1980s, and we came into compliance with the standard at that time. And EPA lowered the standard and we fell out of compliance of that standard. But then we came back into compliance with that standard and EPA lowered that standard again,” Regional Air Quality Council executive director Ken Lloyd said. “Part of this is that we are chasing ever lowering standards.”

But air quality wasn’t the only thing that RealtyTrac looked at.

Denver County had an average of 0.82 Superfund sites on the national priority list per square mile compared to a national average of 0.03

per square mile and an average of 1.58 of other environmental hazards per square mile compared to a national average of 0.09

.

Based on data it obtained from the U.S. Drug Enforcement Administration, RealtyTrac calculated there are 0.2 former drug labs per square mile in Denver, compared with the national average of 0.002 and 0.06 in Broomfield.

Howard Pankratz: 303-954-1939, hpankratz@denverpost.com or twitter.com/howardpankratz