They can be the perfect antidote for a hot summer day: supersize inflatable pools, big enough for adults to swim or even play water volleyball.

They're cheap -- just a couple hundred dollars for a pool that's 12 feet in diameter, or $500 for a 12-by-20-foot pool that's 4 feet deep. And they're relatively easy to set up, taking less than an hour to get ready for water. No wonder such retailers as Wal-Mart, Home Depot, Target and Toys R Us have loaded their shelves with them.

But a growing number of safety experts say consumers are not factoring in the cost of keeping those grown-up kiddie pools safe. These soft-sided watering holes are no different than real pools and should be surrounded by a fence to keep young children away, safety experts say. And the water needs to be kept clean as well, necessitating pumps, filters and chemicals.

In other words, that cheap pool may not be such a deal after all.

"As a whole, these are an unsafe product," said Donald L. Mays, senior director for product safety at Consumers Union, the publisher of Consumer Reports.

The magazine recently issued a safety alert about these pools, as did Good Housekeeping and the Insurance Information Institute, a trade group for the insurance industry.

Unlike kiddie pools, which are usually emptied after each use, these supersize pools -- a 12-by-20-foot pool, 4 feet high, can hold as much as 4,313 gallons -- would be inconvenient to drain between swims. Many of these new pools come with filters to encourage water retention.

Safety officials say any pool not drained after each use -- even those only 2 feet high -- should be surrounded by a 4-foot fence with a self-closing gate. But that requirement would make these pools, which range in price from $50 to $750, cost-prohibitive. Long Fence said it would cost about $1,600 to install a fence around a pool 12 feet in diameter.

"The inflatable pool has the same issue as any pool, and that is, it is an especially fatal attraction to young children," said Mark Ross, a spokesman for the Consumer Product Safety Commission.

This summer, at the request of safety-standard-setting organization ASTM International, the commission staff held a meeting with safety advocates and manufacturers to discuss concerns and possible solutions. Retailers were invited but didn't show up. "No one came up with a viable solution," said Mays, who is co-chairing an ASTM task force to see if voluntary industry safety standards can be developed.

There are no firm numbers on how many of these pools are in use. John Kupsch, technical director of the Good Housekeeping Institute, estimates over 2 million, more than double the number three years ago.

As much as price, the manufacturers stress the ease of setup. One brand, for example, is sold as the "Family Quick Pool," another as "Easy Set." No digging or sand is required, just level ground. Often sold with air pumps, these pools can be inflated within minutes, the packages all say.

In most stores, there are few, if any, cautionary notices that fences may be required by local laws. The pool manufacturers note that they put such advice on their packages. But a quick scan of several pool boxes shows that these warnings are typically at the bottom, in far smaller print than the promotional information. Manufacturers also say they include safety warnings in the instruction manuals and video tapes that come with the pools.

That's little reassurance to Carol Pollack-Nelson, a human-factors psychologist and independent safety consultant who is co-chair of the ASTM task force. "People often refer to instruction manuals only to extent they need to use the product. That means they won't read it from cover to cover, and therefore it's very likely they won't see any warnings or any information about local codes" requiring fences around pools.

The Consumer Product Safety Commission says about 250 children under 5 drown annually in swimming pools; in 2004, another 2,300 were treated in emergency rooms for submersion injuries. After auto accidents, drownings are the second leading cause of unintentional death for young children.

"They are the most susceptible to accidental drowning because children under 5 don't understand the danger a pool poses," said commission spokesman Ross.

The safety commission knows of at least 12 drownings in inflatable pools since 2001, when the pools first became popular. Press reports indicate at least three more within the past month.

"That's not a lot of incidents, which leads me to believe, is there really a problem?" asked Carvin DiGiovanni, head of International Aquatic Foundation, a nonprofit standard-setting organization for the pool industry. The best safety protection, he added, is "constant, competent adult supervision."

Pollack-Nelson, however, said in many cases, the young children who have drowned have been out of sight less than five minutes and were in the care of one or both parents at the time.

Last month, a 21/2-year-old boy drowned in an inflatable pool near Olympia, Wash. He had been out of his mother's sight for about 10 minutes, said Dan Kimball, chief criminal deputy of Thurston County, Wash. The mother said she had taken down the ladder to the pool to prevent the boy from climbing in, but apparently it had been put back up by some neighborhood boys.

On the same day, an 18-month-old girl drowned in an inflatable pool in Houston. News reports said she got out of the family's house by crawling through a pet door.

The ease with which children can climb into the pools is not the only problem, Pollack-Nelson said, noting that the debris and solar covers sold for these pools may exacerbate the drowning risk. Last year, two 9-year-old girls jumped onto a covered inflatable pool 15 feet in diameter. The girls got trapped in the cover, fell into the water and drowned.

This summer, in Quebec, a 13-year-old drowned when her hair got caught in the inflatable pool's filter. Her head was pinned to the side of the pool underwater, and she could not get loose.

Then there are sanitation concerns; although many of these pools come with filters to clean the water, some industry officials say they are inadequate. "The filters I've seen look like a pump for a home aquarium," not a filter that can turn over water every six hours to keep it clean, said Jim Manning, executive director of the United Pool and Spa Association, a trade group for pool contractors and service firms.

Safety concerns were not on Debra Johnson's mind when she bought her 7-by-10-foot pool last year for $69. "It was a good buy and looked easy," said the Alexandria resident, who has two 4-year-olds.

Besides, she said, the 2-foot-deep pool was large enough to hold the entire family. "Since we never leave the kids alone, at least we could sit in the pool and be cool, too."

The pool was set up near the carport, with no surrounding fence, because the water is shallow, and there are no kids who live nearby, Johnson said. "We felt safe." Her main concern, she added, was keeping the pool clean; it was a constant challenge. The pool has since been packed away.

Localities' fencing requirements vary greatly. Some areas don't require any fencing for above-ground pools as long as the pool's ladder can be removed. In Fairfax County and the District of Columbia, however, fences are required for any pool more than 2 feet deep; in Montgomery County, pools over 18 inches deep have to be in rear yards and surrounded by a fence. However, building officials in these three areas say they have received few queries or complaints about fences for above-ground pools.

The growing popularity of these pools will make it difficult to create an acceptable safety standard, acknowledges Mays of Consumers Union. "It will be a technically difficult thing to do without affecting the aesthetics or causing the price to be prohibitively expensive. That's the fix we're in."