It's getting harder and harder to get away from it all. The world has shrunk — and the tourist legions have exploded. The streets of Paris and Venice are so crowded that you can barely move. Cruise ships are filling harbors and disgorging hordes of day trippers the world over. Towering hotels rise in ever-greater numbers along once pristine and empty beaches.

Thanks to globalization and affordable transportation, there aren't many places where you can travel today to avoid the masses of adventure- or relaxation-seekers who seem to alight at every conceivable site. I used to love going back to my old haunt in a Himalayan hill station where, as a student in India in 1970, I climbed those steep, silent paths and watched langur monkeys swinging in the trees outside my window. No longer. Now, Moussurie is chock-a-block with tourist lodges, garbage and noise; the monkeys are fleeing.

This problem goes far beyond a veteran traveler's complaint that things aren't the way they used to be. What's happening today is of another magnitude.

The places we love are rapidly disappearing. Global tourism today is not only a major industry — it's nothing short of a planet-threatening plague. It's polluting land and sea, destroying wildlife and natural habitat and depleting energy and natural resources. From Asia to Africa, look-alike resorts and spas are replacing and undermining local culture, and the international quest for vacation houses is forcing local residents out of their homes. It's giving rise to official corruption, wealth inequities and heedless competition. It's even contributing to human rights violations, especially through the scourge of sex tourism.

Look at Cambodia. The monumental temples at Angkor and the beaches on the Gulf of Thailand have made that country a choice destination, especially for Asians, who spent $1 billion there last year. But the foundations of those celebrated temples are in danger of sinking as the 856,000 tourists who every year crowd into Siem Reap, the nearby town of 85,000, drain the surrounding water table.

And at night along the riverfront in the capitol of Phnom Penh, the sight of aging Western men holding hands with Cambodian girls young enough to be their granddaughters is ugly evidence of the rampant sex-tourism trade.

All this came as a shock to me. I've been writing about Cambodia for more than 35 years, but I never considered tourism there a serious subject. But when I went back last November, I couldn't avoid the issue. In three short years, tourism had transformed the country. In every interview, the conversation wandered toward tourism, its potential and its abuses. When I went up to Siem Reap, I found the great hall temple of Angkor as crowded, as a colleague said, as Filene's Basement during a sale. Forget tapping into any sense of the divine.

Most people can't imagine that tourism could be a global menace. Travel has always been surrounded by an aura of romance. For centuries, beginning with the first tourists on holy pilgrimages, travel has been about adventure and discovery and escape from the pressures of daily life.

It wasn't until the end of the 20th century that tourism was added to the list of industries measured in the U.S. gross domestic product. And the results were a revelation: About $1.2 trillion of the $13 trillion U.S. economy is derived from tourism.

Tourism has become the stealth industry of the global era. According to the United Nations, the international tourist count in 1960, at the dawn of the modern era of air travel, was 25 million. By 1970, the figure was up to 165 million. Last year, about 898 million people traveled the globe, and the international tourism industry earned $7 trillion. (Figures don't include people who vacation in their own countries.)

The U.N. World Tourism Organization was established as a special agency five years ago with the twin goals of keeping track of the tourism industry and figuring out how poor countries, in particular, can take advantage of the tourist boom without causing their own ruin. Geoffrey Lipman, the assistant secretary-general of the new organization, has spent his life studying the industry. "If properly conducted — maintaining respect for a country's environment and culture, providing local jobs and a market for local goods — tourism, the United Nations believes, is easily the best way for a poor nation to earn foreign currency, he told me.

There are several promising examples of this philosophy at work. The nonprofit British National Trust offers tourist rentals in restored cottages and historic mansions, and then uses the money to buy more land and properties to preserve and protect. The African nation of Namibia, meanwhile, has created what it calls "community-based tourism," which manages more than 25 million acres of wildlife preserves, opening much of the land to tourism — hunting or photo safaris, birding and white-water rafting — that employs local residents and has dramatically reduced poaching.

Most of the tourism industry, however, is heading in the opposite direction. Tourism is now responsible for 5 percent of the world's pollution, according to a recent study. Cruise ships are one of the biggest culprits, creating three times more pollution per passenger mile than airplanes. Years of cruises have helped spoil the water of the Caribbean, which, according to the United Nations, absorbs half the waste dumped in the world's oceans. Now these ships are venturing into fragile polar waters. Last year, Norway banned all cruise ships from visiting its region of the Arctic Circle.

Beach erosion has been swift. After the Indian Ocean tsunami in 2004, fishermen were told to move their homes away from the beaches, but luxury hotel chains with clout were allowed to rebuild near the water's edge. In the U.S., the upswing in violent hurricanes hasn't put a dent in the number of vacation homes being built by the sea. "Essentially every tropical island is in danger," the National Geographic Society's Jonathan Tourtellot told me.

In poorer nations, unregulated tourist developments have put unbearable strains on scant resources, especially water. High-end tourists often waste more water in a day with frequent showers and toilet flushes than some local families use in a month.

Questions raised by mass tourism have become too large to ignore. Isabel Hill, director of the Commerce Department's Office of Travel and Tourism Industries, hopes that the U.S., like so many European countries, will "recognize our limitations and how we have to regulate our resources."

Still, there probably won't be a U.S. secretary for tourism and the environment anytime soon. But don't be surprised if the next international agreement on climate change mentions the role of tourism, or if some countries start regulating tourism along with the environment, because the two go hand in hand.

In fact, you'd better hope that they do — if you ever again want to find that cool vacation spot where you can get away from it all.

Elizabeth Becker, the author of "When the War Was Over: Cambodia and the Khmer Rouge Revolution," studied media coverage of tourism at Harvard's Joan Shorenstein Center on the Press, Politics and Public Policy. Reach her at ehb47@msn.com.