Fire engines in Prince George's County are taking longer to respond to emergency calls and are missing the county's target for responding to fires in six minutes or less.

Response times at fire stations across the county rose by 20 seconds from fiscal 2005 to fiscal 2011, from an average of 6:36 to 6:56, according to a Prince George's CountyStat report. Since 2005, only 42 percent of fire engines have been responding to emergencies within the county's six-minute goal, down 9 percentage points. At the same time, the number of emergency calls per engine has dropped by nearly 100.

But fire officials challenged the standards set by county researchers -- the Prince George's Fire and Emergency Medical Services Department uses different benchmarks set by the Maryland-National Capital Park and Planning Commission in its 10-year public safety master plan, which sets goals of seven-minute response times in developed regions and nine minutes in rural areas.

Because fire stations serve such a diverse county, with a large rural region outside the Capital Beltway but also densely populated, developed areas near the D.C. border, it's unreasonable to look at the county's response times as a whole, said Fire/EMS Department spokesman Mark Brady.

"If you do, holistically, look at the whole county, it's impossible to meet those response times," he said.

The six-minute figure used by county researchers comes from research by the National Fire Protection Association.

The county's own data show that long response times in rural regions are helping to drag down the county's overall average. CountyStat officials recommended the department hire recruiters to bring in volunteer firefighters to help staff stations at rural areas to lower response times.

Prince George's hired 80 firefighters in fiscal 2012 and has enough money budgeted to hire 60 to 80 firefighters in fiscal 2013, Brady said.

"Having additional volunteer firefighters will help, but that's a very long process," Brady said. "We'd always like to have more, but what we have is working for now."

In a memo to Fire Chief Marc Bashoor, CountyStat director Adam Ortiz asked the department to study stations with slow response times, as well as look into six stations that have high "failure to respond" rates -- instances when emergency calls go unanswered and must be redispatched to the next-closest emergency response team.

Those stations are Capitol Heights, Riverdale Heights, Boulevard Heights, Upper Marlboro, Bowie and West Lanham.