The early Saturday accident — bizarre but not unprecedented — caught airport workers by surprise, even though the security line was not busy at the time, officials said.
The infant was taken to Centinela Hospital, where doctors determined that he had not received a dangerous dose of radiation.
Officials, who declined to release the 56-year-old woman's name, said she spoke Spanish and apparently did not understand English.
She initially didn't want the baby transported to a hospital, but security officials called paramedics and insisted that the child be examined by a doctor.
The grandmother and the child were subsequently allowed to board an Alaska Airlines flight to Mexico City.
The rare incident drew attention to whether officials are staffing often-busy security checkpoints enough to prevent such an accident. And it raised questions about the danger of X-rays used to pick out suspicious metal shapes in passenger bags, given the medical community's warnings that even low amounts of radiation can build up over a lifetime.
"Rather than focus on the radiation dose, which is a small amount, we need to focus on why this happened, so it doesn't happen again," said Dr. James Borgstede, a diagnostic radiologist at Penrose-St. Francis Health Systems in Colorado Springs, Colo., and president of the American College of Radiology. "Human beings weren't meant to go through those things."
In the several seconds the baby spent in the machine, the doctor added, he was exposed to as much radiation as he would naturally get from cosmic rays — or high energy from outer space — in a day.
Security experts said the incident underscored a more widespread concern about the screening process at LAX and other airports.
"The screeners are still reporting that they're being pushed," said Brian Sullivan, a retired Federal Aviation Administration security agent. "If a baby can get through, what the hell else can get through?"
Nico Melendez, a spokesman for the Transportation Security Administration, which manages LAX screeners, said the agency doesn't have enough workers to constantly stand at tables in front of the screeners to coach passengers on what should or should not be sent through X-ray machines.
But in some cases, airlines contract with private companies to staff the tables and assist travelers. The TSA will also occasionally put employees at the tables if extra workers are available.
TSA screeners often ask passengers to put their coats, shoes, laptops and other items into the bins, Melendez said. But they cannot observe everything people place there, because they must monitor screening equipment, he added.
Still, he said that the TSA works hard to educate passengers about what carry-on objects require screening and that travelers must take responsibility for knowing these rules.
"There's an obligation on the traveler to use some common sense," said Larry Fetters, the TSA's federal security director at LAX. "If they don't understand, they should ask somebody. If they ask us, we are generally able to find someone who speaks that language and assist them."
On its website, the TSA posts extensive tips for travelers, including a section titled "Traveling With Children."
One item reads: "Never leave babies in an infant carrier while it goes through the X-ray machine."