Rapid growth in the multi-billion dollar volunteer tourism industry has prompted calls for tighter controls with concerns over exposing vulnerable communities to unskilled foreign labor and dodgy operators exploiting foreigners for profit.

Voluntourism, which allows socially-conscious holiday-makers to pay thousands of dollars to work in poor communities across South America, Asia and Africa, has become a boom sector of the global travel industry.

Estimates of its size vary widely. Nancy Gard McGehee, an expert on sustainable tourism at the U.S. university Virginia Tech, says as many as 10 million volunteers a year are spending up to $2 billion on the opportunity to travel with a purpose.

Carnival Corp., the world's largest cruise operator, this month announced a "social impact" cruise which allows travellers to take part in three days of volunteering, helping to cultivate cacao plants, building water filters and providing English tuition.

But with no industry regulator, campaigners within the sector are concerned about the rising numbers of companies involved, with no mechanism to hold them to account for the work that they do.

"One of the challenges facing people wishing to volunteer responsibly is that there is no independent quality standard, no recognised regulatory body," said Simon Hare, development director of British charity Globalteer.

"There are small local outfits as well as big corporations who see volunteering as away of driving profits rather than an integral part of a long term strategy for communities with real needs. At best this can make volunteering a waste of time and at worst it can actually be harmful."

Critics warn the lack of oversight means volunteers can easily end up in parts of the world without the skills needed to help, take away local jobs, and form bonds with children in need that are short-lived as they quickly move on.

In the wake of the April 25 earthquake in Nepal, the United Nations children's agency, UNICEF, said it became alarmed by reported cases of child trafficking, calling on orphanages and volunteer agencies to stop sending more willing workers.

"We would ask people to consider carefully the impact of volunteering or donating funds to post-earthquake Nepali children's homes in Kathmandu. Without realizing it, such support may be indirectly harming children," UNICEF said.

Seeking foreign funds

UNICEF said it had encountered the same problem in Cambodia, where there has been a rise in the number of unregistered childcare institutions, kept afloat by the funds and steady influx of volunteer tourists from abroad.

"Many volunteers have absolutely no childcare skills, and they're being asked to perform a duty of care for children who are vulnerable. In a developed country, that would not happen," said James Sutherland from Friends-International, a children's charity based in Southeast Asia.

Australian academic Nichole Georgeou, co-author of "Looks good on your CV: the sociology of voluntourism recruitment in higher education", said part of the problem was that the industry is consumer driven rather than driven by the needs of the local communities involved.

"There's this idea that is in-built in voluntourism that we in the West have the knowledge and the skills to make a difference, we have a right to make a difference," said Georgeou from the Australian Catholic University.

"It doesn't even matter if we're unskilled, it's just the good will that matters because we're somehow bonding anyway."

A recent study by Britain's Leeds Metropolitan University, published in the Journal of Sustainable Tourism, warned students considering a project for a gap year or summer break that the most expensive trips were found to be the "least responsible".

Authors Victoria Smith and Xavier Font said volunteer tourism organizations needed to take more responsibility.

"These organizations have a responsibility to ensure their programmes have positive and not negative impacts and should offer financial transparency," said their report.

"This means proper needs assessments, appropriately recruited, matched and skilled volunteers working with locals, with clear objectives, sustainable programme management, reporting and lasting impact and respect."

Some returning volunteers have expressed their concerns about the negative impact they might have had.

"The kids [in the orphanage] were so used to seeing volunteers that they were barely paying attention to us," said Carla Salber, who volunteered in Cambodia with Projects Abroad, one of the largest voluntourism companies. "We felt betrayed."

Voluntourism proponents dispute the claim that the industry is doing more harm than good, citing numerous schools and homes that would not have been built without voluntourists and their funding.

"The idea that people shouldn't come at all in case they traumatize a child who had the most terrible in their life already is really verging on the ridiculous. All our volunteers want to do is help," said Peter Slowe, founder and director of voluntourism provider Projects Abroad.

Globalteer's Hare said it was a mistake to lump together good volunteering with bad volunteering and call it all "voluntourism".

"This is a shame because there are organizations running really impactful volunteer programmes," he said. Regulation of the industry was the next step.