ALPHARETTA, Ga. -- The number of evangelical Christians volunteering as missionaries in the United States and overseas has more than doubled since 1997, according to the North American Missions Board.

"We project there were more than a half-million missionary volunteers last year, just in the United States," said Jim Burton, director of volunteer mobilization for NAMB, which is part of the Southern Baptist Convention.

The Dallas Morning News reported that in 1997, 210,748 Southern Baptists volunteered for missions in the United States and Canada.

Affordable air travel, populist volunteerism and immigration contributed to the growth, but Burton said many were simply following their hearts.

"This kind of work answers the question of how significant people feel their lives are," Burton said. "Not only are people asking, ‘What am I doing? But ‘What have I really done?' This experience gives definition and meaning to people's lives."

John Fletcher, international director of Pioneers, a Florida-based missionary organization, told the newspaper building houses, medical clinics or preaching in the inner city strikes a chord with teenagers, young adults and people approaching retirement.

"Short-term missions really benefit the individual most," Fletcher said. "Hopefully, the work … meets needs and benefits the local culture, but it provides the people who come home a richer and deeper understanding of diversity and struggles in the world."