How many 3oz bottles can i carry on

SIX years into heightened airport security, most of us agree that the checkpoint experience has become quicker, more convenient and less arbitrary than it once was.

Part of the reason, of course, is the comparison with the subsequent airline experience, with its record delays and cancellations.

Nevertheless, the customer service aspect of the security process has evolved, and the Transportation Security Administration deserves credit, as I said in this space not long ago.

But not so fast, some readers like Alicia Zadrozny replied.

“I don’t want you to give the T.S.A. too much credit yet,” she wrote, echoing the sentiments of a few others who say that the checkpoint experience, while vastly improved, can still come with stultifying doses of arbitrariness.

Much of the current discontent comes from enforcement of the requirement that various liquids and gels carried on a plane must be in individual containers of three ounces or less. That rule was imposed after terrorists in Britain tried to sneak liquid explosives in planes in August 2006.

The security administration’s rules on liquids are clearly spelled out on its Web site, www.tsa.gov (Click on “For Travelers” and then “3-1-1 for Carry-ons.”) The rules state that a passenger may carry on liquids and gels only in separate three-ounce containers in a single quart-size plastic bag, with a limit of one bag for each passenger.

But, as they say in the Army, there are always about 10 percent of the troops who don’t get the word. Ms. Zadrozny, an editor with Weekly Reader Publishing, found that out while planning a trip with some friends recently.

Before the trip, there was much discussion about the liquids rule. Several of her friends purchased generic plastic three-ounce containers for their shampoos and other liquids. But one friend said that her liquids had been confiscated at a checkpoint recently because they weren’t in containers with “manufacturers’ labels,” Ms. Zadrozny said in an interview.

Not wanting to have her toiletries tossed out, she assembled some generic three-ounce containers and assiduously spent time pasting manufacturers’ labels on them.

“It was a little insane, but still, I couldn’t believe how the labels stuck,” she said. “If you looked closely you could tell the labels weren’t exactly straight and there was missing information. But I thought, like, who’s going to know from a label anyway?”

Nobody. Actually, despite what a few uninformed checkpoint screeners might say, there is no requirement that a container carry a manufacturer’s label.

In fact, said Kip Hawley, the director of the security agency, the size of the container is more important from a security perspective than what’s in it.

In an interview, Mr. Hawley explained that security officials in the United States, Britain and elsewhere quickly determined that the liquids explosion plot was “chillingly real.”

In August 2006, the thwarted terrorists were planning to use “a formulation that was specifically put together to get around our screening measures,” he said. In response, officials banned liquids altogether and then, after laboratory testing here and abroad, came up with the formula of limiting liquids and gels to three-ounce containers in a single quart-size bag “as a way to allow for people who wanted to bring liquids on board,” Mr. Hawley said.

“We didn’t want to be in a position of opening all those bottles,” he said.

Tests showed that a container of a certain size is needed for an effective explosion. Separate three-ounce containers limited in number to what will fit inside a single one-quart bag do not have “enough critical diameter” to blow up an aircraft, he said.

The rule was ridiculed. Critics scoffed, Mr. Hawley said. “Holy smokes! Three ounces in a one-quart baggie! Who made that one up?”

But the science, he said, is clear. “With certain explosives you need to have a certain critical diameter in order to achieve an explosion that will cause a certain amount of damage.”

“The size of the container itself,” he added, “is part of the security measure.”

He added: “It is incredibly complex and doesn’t lend itself to a sound bite. And we’ve certainly paid the price for that.”