Millions of homes built on the edge of wild areas are complicating the work of wildfire managers who must decide how to deploy the nation's firefighting resources.

Some 44 million homes have been built in what scientists call the wildland-urban interface, most in recent decades. The areas are often scenic but susceptible to fire.

Protecting those homes from flames is expensive and drains resources that might otherwise protect forests, rangelands and critical habitat for wildlife. Besides the financial burden, there's also the human cost. Three firefighters died in Washington state last year, and 19 perished in Arizona in 2013 while working in those areas.

Fire officials emphasize the need for homeowners to put so-called defensible space between their homes and vegetation to protect against wildfires.