The unusually high number was due in part to the long stretch of hot weather. Temperatures had topped 90 degrees for nine days before the week began, reaching 106 degrees July 22 at Baltimore-Washington International Thurgood Marshall Airport and 101 degrees July 29.
A review of last year's heat-related deaths found that most involved people "who do not have air conditioning at home and who do not leave home," Phillips said.
All of the people who died in last week's heat had underlying health conditions that made them more vulnerable, she said.
"We were expecting we would see deaths," Phillips said. "We were surprised at the actual number."
Despite efforts to make sure that communities have cooling centers and that proper alerts go out, she said, "it's disturbing to see we still have a great deal of work to do to avoid preventable deaths."
The determinations that the victims had underlying health conditions were made during autopsies by the office of the chief medical examiner.
Phillips said the high death toll last week has prompted the Department of Health and Mental Hygiene to launch a study of how many of the victims knew of their underlying health problems.
"It's important that people who are middle-aged and older not disregard [weather] warnings," she said. "They could have conditions that have not been diagnosed — circulatory disorders or hypertension. It's not unusual to have these problems and not have symptoms."
If it turns out that a significant percentage of the dead were unaware of their underlying illnesses, she said, health officials might need to change the warnings they issue during heat waves.
"Heat is not a condition to underestimate," Phillips said.
The heat contributed to the deaths of 10 men and one woman last week. Four died in Baltimore City, two in Anne Arundel County, two in Wicomico County and one each in Baltimore, Cecil and Worcester counties.
The woman and six of the men were 65 or older, health authorities reported. The other four were between the ages of 45 and 64.
A woman who died earlier this summer was added to the list released Tuesday after the cause of death was confirmed. Her age was listed as between 18 and 44.
The health department's "Weekly Heat-related Illness Surveillance Report" said 332 people reported to emergency rooms last week complaining of heat-related illnesses. The traffic peaked at 67 patients Tuesday, when the temperature at BWI reached 95 degrees. It rose again to 55 patients on Friday, when the high was 101 degrees.
Health authorities reported 32 confirmed heat-related deaths in 2010, the hottest summer on record in Baltimore. There were six deaths in 2009; 17 in 2008 and 21 in 2007.
Last month was the hottest on record for Baltimore, with an average temperature of 81.7 degrees. Twenty-four days reached 90 degrees or more, a record for any July.
Wednesday was the first day since July 16 that failed to reach 90 degrees.
Maryland weather blog: Frank Roylance on meteorology
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