In the wake of the disaster in Worcester, Mass., last December -- when six firefighters died in an empty and burning warehouse -- city officials have suddenly become concerned over the perils posed by abandoned properties.

For most of the past decade, more firefighters have been injured or killed in vacant buildings than any other type of fire, experts report.

• As the economy has shifted from manufacturing to services, countless factories and warehouses in cities sit empty or crumbling -- and have become havens for the homeless who often create fires for warmth during winter months.

• While there is no national census of vacant and abandoned buildings, researchers at Ohio's Miami University and the University of South Carolina estimate that an average of 18 percent of urban structures sit unused, based on a survey of 100 cities.

• About one-third of the cities responding to the survey couldn't even provide an estimate.

• The Insurance Services Office figures that 21,000 buildings of 15,000 square feet or more sit empty across the country.

Former factory towns in the Northeast have the highest concentration of abandoned buildings, but residents in Western states seem to be the most creative in tackling the problem. They often employ restoration as a solution.

Seattle's old warehouse districts are being converted into residential and commercial properties. Houston -- which was left with 8,000 abandoned buildings after its last economic turndown -- has seen many of them in downtown converted into law firms and offices.

Source: Fred Bayles, "Nightmare of Buildings that Stand Empty Haunts Cities," USA Today, March 20, 2000.