This story was updated Dec. 29 at 12:58 p.m. EST.
Each winter, many people decorate their homes and offices with holiday lights, ornaments and tinsel. They chop down fir trees, and sit near them at home, breathing in the sharp evergreen scent.
But while many people use the holiday season to relax and recharge their batteries, sometimes these festive pastimes can be downright dangerous, as people — and cats, dogs and owls — have learned over the years.
Here are six examples of when holiday decorations turned into disasters. [7 Holiday Stress Busters]
1. Holiday lights trap owls
Owls and other wild animals can get caught in a bind if they fly into netted holiday lights. People often drape netted lights over trees and bushes, which are "problematic because the lights are there, but the netting is not as visible," said Julia Ponder, executive director of The Raptor Center at the University of Minnesota.
Regular lights lining houses often aren't a problem, and people shouldn't hesitate to decorate if it's a holiday tradition. Nonetheless, Ponder encourages people to take down netted lights when the holiday season ends on New Year's Day.
"The longer they're up, the more chance it's going to be a problem," Ponder told Live Science. "If [owls] are going to roost in a tree or they're chasing a bird for food, they're not expecting to fly into these man-made objects."
Large owls, such as barred owls and great horned owls, are typically the ones that get caught in netted lights, and birds that become trapped in netting are usually traumatized and may have broken bones or soft-tissue damage, Ponder said.
Experts at The Raptor Center often treat injured owls with anti-inflammatory medication, and give them a warm and safe place to recover.
"If you have an injury and you can't hunt, you're going to starve," Ponder said. "Sometimes all they need from us is time from Mother Nature to heal their injuries."
2. Pets may eat holiday ornaments
Fido and Fluffy may get the nibbles when they're around the holiday tree.
"Cats and dogs can get into a lot of mischief — that's for sure," said Karl Jandrey, an associate professor of clinical small animal emergency and critical care at the University of California, Davis. [Images: Early Christmas Cards from Krampus to Smoking Santa]
A bulky ornament made of dough might end up in a pet's stomach, but long strings, ribbons and tinsel can be even more worrisome. "One end [of the string] will get stuck somewhere, and the other end will work its way down the intestine," Jandrey said.
The string can turn the intestines into an accordion, and painfully saw through the intestine, leaking its contents into the animal's abdomen. "If the intestinal contents have leaked into the abdomen, that can make the entire belly inflamed and infected," he said.
Concerned pet owners should look for signs of anorexia, vomiting, diarrhea and abdominal pain. An X-ray or ultrasound at the vet's office can verify whether a pet ate something dangerous. Sometimes, the object will naturally pass through the animal's system, but other times, surgery may be needed, Jandrey said.
If an owner catches a pet in the act of eating an ornament or a string, a vet can quickly use an endoscope — a flexible, wirelike instrument with a light on its end — to pull out the foreign object.
"I've seen dogs get into garbage and take strings wrapped around the roast turkey or chicken," Jandrey said. "The dogs find the flavor amazing, and they don't know about the string. [But] cats are probably a little bit more the culprit for string foreign bodies because of their nature."
Pet owners should also keep dogs away from chocolate, and pets away from burning candles and fireplaces. Moreover, mistletoe, poinsettias, holly and lilies can have mildly irritating to deadly effects on pets that eat them.
3. Inhaling a Christmas tree
Whether a Douglas fir or a Norway spruce, Christmas trees smell like fresh pine needles. But some children may inhale a bit too deeply when they're next to the tree, and pay for it dearly.
A 2-year-old boy visited the hospital because of recurrent pneumonia that had bothered him since the age of 10 months, a few months after his first Christmas. Each time, doctors prescribed him antibiotics, and the pneumonia cleared up, according to a 2004 study published in the Canadian Medical Association Journal.
But the pneumonia continued to return, so doctors gave him a chest CT scan. They found a lesion in his right lower lung, and decided to remove it. The procedure worked, and the boy has not had pneumonia since, the researchers said. [Gasp! 11 Surprising Facts About the Respiratory System]
An examination of the removed tissue found a curious object: a small, green plant that resembled the branch of an evergreen tree, said the study's lead researcher, Natalie Yanchar, an associate professor of surgery in the Division of Pediatric Surgery at Dalhousie University, in Halifax, Nova Scotia.
"We call it a Christmas-treelike structure," Yanchar said. "It looked like a little branch from a Christmas tree."
Other, similar cases have been reported. Another 2-year-old child with noisy breathing and hoarseness had an even larger object lodged in his voice box. Doctors removed a flat, plastic Christmas tree that the child had swallowed, according to a 2004 study published in the journal Pediatric Anesthesia.
The boy's family remembered using the Christmas decoration two years prior, and recalled that the boy started coughing around that time, the researchers said.
The boy's voice improved within 12 days of the surgery, they added.
4. No equipment is safe from tinsel
Some touch-screen instruments in doctors' offices don't get along with festive decorations, according to a 2012 snippet from the journal Anaesthesia.
A clinical technologist at Craigavon Area Hospital in Northern Ireland noticed that the touch screen on a blood gas analyzer wasn't working. "Many years' experience had furnished him with an encyclopedic knowledge of all the kit’s and the staff’s foibles," the researchers said in the piece. "Deft removal of tinsel (which was in contact with the analyzer's screen) restored it to full working order."
The machine's interface used infrared touch-screen technology and a Cartesian coordinate system to determine the position of the user's hand. The tinsel led to "multiple touching," on the touch screen, hence the instrument error, the researchers said.
5. Handsaw disaster
Cutting down a Christmas tree with a handsaw can result in some unexpected injuries, according to a 1994 study published in the Southern Medical Journal.
In December 1992, a 58-year-old man injured himself on an outing with his family to saw down a tree, said Dr. Scott Davies, chief of medicine at Hennepin County Medical Center in Minnesota. After the man found the perfect fir, he lay down on the ground, on his right side, and started sawing with his right hand.
The man held his head off the ground and twisted it to the left to give himself some leverage to cut the tree, but he felt a pain in the right side of his neck. The pain intensified as he continued sawing, but he was almost done, so he finished the job.
The discomfort persisted over the next few days, causing a stiffness and pain in the right shoulder and arm. Some nights, he had trouble breathing on his back, and tried sleeping in a chair instead. Heat and painkillers helped a little, but an X-ray of his chest found the real culprit: his right diaphragm — a muscle that sits below the lungs and helps people breathe — was elevated. Moreover, later tests showed it was paralyzed.
An unrelated X-ray taken nine months before showed that both of the man's diaphragms were the same length before the accident.
Davies explained how the injury likely happened. "Naturally, as you push the repeated saw strokes, the muscles in your neck start to cramp up," he said. "You're twisting your neck away from the ground to get leverage to saw the tree. Normally, if it starts to hurt, you stop."
But the man persisted, and his position put pressure on one of the body's two phrenic nerves, which start in the neck and help control the movement of the diaphragm.
"It's a very odd, peculiar way of injuring your phrenic nerve," Davies said. "It's probably never happened to anyone else."
The man's other phrenic nerve was fine, allowing him to breathe with the left side of his diaphragm.
"He got the Christmas tree, but he would have been better off to cut it down with a chain saw," Davies said.
6. Do Christmas lights contain lead?
The short answer is yes, according to a 2008 study published in the Journal of Environmental Health.
Lead is often used as a stabilizer in the plastic polyvinyl chloride (PVC) jacketing that covers conductors used in Christmas lights and other wires, such as appliance cords and telephone wire. Lead can give PVC resistance to heat, light and moisture.
But it's also poisonous, and can accumulate in a person's teeth and bones. Stress, disease and aging can cause lead to leach into the blood and cause lead poisoning, which is linked to mental and developmental problems, and death.
"Lead, a toxic element that causes serious health problems, is present in the material that covers electrical wires in strings of Christmas lights," study lead researcher Joseph Laquatra, a professor of design and environmental analysis at Cornell University, told Live Science in an email. "People should be aware of the potential of lead poisoning from handling Christmas lights."
The researchers took wipe samples from both new and old Christmas lights, and analyzed them for lead content. All of the samples contained lead, but in varying concentrations, including some in excess of the U.S. Environmental Protection Agency's allowance for lead on windowsills, the researchers found.
It's not clear whether exposure to lead in Christmas lights affects blood lead levels in people, the researchers said, but decorators can still take precautions.
"Wash your hands with soap and water after handling Christmas lights," Laquatra said. "And be aware that Christmas lights that are put outdoors can degrade in sunlight and can contaminate soil."
Editor's Note: This story was updated to fix several grammatical errors.
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Laura is the archaeology and Life's Little Mysteries editor at Live Science. She also reports on general science, including paleontology. Her work has appeared in The New York Times, Scholastic, Popular Science and Spectrum, a site on autism research. She has won multiple awards from the Society of Professional Journalists and the Washington Newspaper Publishers Association for her reporting at a weekly newspaper near Seattle. Laura holds a bachelor's degree in English literature and psychology from Washington University in St. Louis and a master's degree in science writing from NYU.