Every winter morning as my kindergartner and I head for the bus stop, I hear the same lament: "Do I have to wear my heavy coat?" I know I'm not the only parent to have heard this kind of protest, but as a science writer I can offer an evidence-based response beyond "Yes, because I said so."
Are we over-bundling our kids when we stuff them into those big parkas? How warm need a coat be? And what's the science behind those temperature ratings on many kids' jackets?
First things first: Kids generally run hotter than adults do. The metabolic rates for kids up through their teens are higher than those of most adults, and so are their activity levels. As a result of both these factors, kids tend to generate more heat than adults do.
But the under-12 set also has more surface area relative to their mass, which means they lose body heat faster in cold temperatures. Kids also have a less-developed system for regulating body temperature and tend to have less natural insulation, the fat layer just under the skin.
"I think of them like little hummingbirds," says Deborah Mulligan, a professor of pediatrics at Nova Southeastern University in Fort Lauderdale, Fla. "They are burning through calories at a higher rate, which renders them more sensitive to extreme temperature changes. You have to make sure they are replacing that fuel, with food and drinking."
The net effect is that, yes, kids are more vulnerable to losing body heat than adults and should be bundled properly in jackets, gloves and hats. Indeed, the adage about losing more heat from the head is true, says Lee Beers, a pediatrician at Children's National Medical Center.
"A hat can go a long way toward keeping your child warm," Beers says. "If you are going to fight over something, that might be the thing to fight over."
So, at what temperatures should parents enforce coat -- and hat -- wearing?
Paul Stricker, a youth sports medicine specialist at the Scripps Clinic in San Diego, advises bundling up for temperatures (including the windchill factor) between 10 and 32 degrees. If windchills dip below 10 degrees, it's best to stay indoors to play.
Hypothermia can be a risk for young athletes playing outdoor fall or winter sports, Stricker says. A child who has gotten hot and sweaty, then suddenly gets taken out of the game may experience a rapid drop in body temperature. "The child may feel hot," Stricker says. "But they need to avoid cooling off too quickly. They should peel off layers gradually."
So what are the best coats for that outer layer?
"By far, down is nature's best insulator," says Todd Christiansen, kids merchandise manager for Lands' End in Dodgeville, Wis. Elizabeth McCullough, a textile scientist at Kansas State University, explains that this is because down provides the most warmth -- by trapping air between the goose feathers -- for the least weight. And down is the most compressible insulator, giving kids less restriction. But down also loses insulating ability when wet, and it's expensive. Synthetic materials, such as poly-fill, are cheaper insulators that still work when wet.
McCullough, who developed the mathematical model used by companies such as Lands' End and L.L. Bean to rate the warmth of kids' coats, says that the best design sandwiches the insulation between a waterproof, windproof outer shell and an internal liner.
McCullough's conclusion comes from testing on "STAN," a computer-operated mannequin whose molded plastic shell is fitted with 20 thermal zones, each with a heater to simulate metabolic output rates. McCullough dresses STAN in a winter outfit plus a parka and uses a fluid running through tubes to heat him to 95 degrees, the average human surface temperature. She then chills the air in the mannequin's sealed room to 41 degrees and measures how much power it takes to maintain STAN's surface temperature. That tells her the insulation value of the garment, which allows her to determine the temperature range in which the garment will keep a wearer comfortable.
By taking into account children's particular physiology -- higher surface-area-to-mass ratio and metabolic rate, lower body temperature and blood volume, and a decreased sweating response -- McCullough's team came up with a formula to set temperature ratings for a parka worn by a 4-year-old and another for a 10-year-old. A 2009 study using these calculations indicated that "the base ensemble" -- the clothes worn under the parka -- "had a major effect on jacket ensemble insulation and the predicted comfort temperature" of the wearer.
In other words, she explained later, a one-inch layer of good insulation covering the whole body is better than a four-inch puffy coat that covers only a child's torso: "The best way to protect children is to protect all body parts as evenly as possible."
As any parent knows, that's easier said than done.