The Twin Cities area experiences more days of extreme heat and humidity and more heat waves each year now than in the 1940s, raising implications for public health, according to a study released Wednesday by the Union of Concerned Scientists [UCS].

In particular, the warmest, most humid nights have gotten more so, giving vulnerable residents, particularly the poor and elderly, less chance to find relief, says the study from the Cambridge, Mass.-based organization.

The findings are based on a review of Twin Cities National Weather Service data from 1945 to 2011 and are part of a report that also looked at four other large Midwest cities.

The study found that the Twin Cities area has 4.5 more days a year with the types of air masses most harmful to human health -- hot and dry, and hot and humid. Hot and dry are days with 3 p.m. temperatures of roughly 92 and dew points of 59; hot and humid days have 3 p.m. readings of roughly 90 and 71, respectively.

The Twin Cities also experience each year, on average, one more heat wave -- defined as three or more days under those air masses -- than 60 years ago, the researchers said.

Three days is a somewhat arbitrary period, but one after which heat-related deaths begin to rise, said Laurence Kalkstein, professor of geography and regional studies at the University of Miami and lead author of the study. UCS Midwest office director Steve Frenkel, citing the Chicago heat wave of 1995, in which more than 700 people died of heat-related causes, noted that heat is the leading cause of weather-related deaths in the United States.

In Wisconsin, at least eight people died of heat-related causes in the first three weeks of July this year, and 11 other deaths might also have been heat-related. Statistics for this year aren't available in Minnesota, said Kristin Raab, environmental research scientist for the state Health Department. But Minnesota had 35 deaths between 2000 and 2010 in which the primary cause of death was heat.

The department is reviewing its death records and consulting with Wisconsin. "We don't know what the numbers are," Raab said of Minnesota's heat-related deaths. "We know people are dying from extreme heat. The fact that Wisconsin had those deaths, it's very, very likely we had similar amounts here.'

This summer has seen 24 days with highs of 90 or higher in the Twin Cities. The average is 13, and the record is 44, set in 1988.

The UCS study also showed that the long-term warmup was more pronounced in some ways in other cities studied. Already-warm St. Louis had increases of 10 extremely hot, humid days and of four heat waves per year. Chicago, Cincinnati and Detroit were the other cities studied.

The study praised the Twin Cities area for its heat emergency planning, including the promotion of air-conditioned buildings open to the public, and the mapping of vulnerable populations. The percentage of owner-occupied Twin Cities homes with central air conditioning jumped from 42 percent in 1985 to 84 percent in 2007, according to the Minnesota state demographer.

Frenkel said increasing warmth also underscores the need for a shift toward energy sources other than fossil fuels, the burning of which creates gases that trap warmth in Earth's atmosphere.

The report can be accessed at

Bill McAuliffe • 612-673-7646