Science explains how a Christmas tree turns into a deadly inferno

A single spark from a broken string of lights or a jumping flame from a fireplace can transform Christmas trees from magical decorations into raging infernos. The holidays are a notoriously bad time for fire, and Christmas trees are a frequent cause.

There are about 160 Christmas tree fires each year, according to the National Fire Protection Association (NFPA), making up a tiny percentage of total house fires.

“Christmas tree fires don’t happen very often. But when they do happen, they’re much more likely to be serious, and by serious, I mean deadly,” Susan McKelvey, a communications manager at the NFPA, tells Inverse.

Christmas trees may seem innocuous, but as the NFPA’s video demonstrates, if they do catch fire, you’re in serious trouble.

“They burn so quickly. It’s not a long smoldering fire; they become very fast, serious fires in a very short window of time”

The illusion of safety

During the holidays and beyond, many people maintain the illusion of safety when they’re in their own home. Home is where people feel safest from fire, McKelvey says, but it’s the place where you are actually at greatest risk.

A home fire occurs every 87 seconds in the United States, according to the NFPA. Seventy four percent of all fire-related deaths happen in a home, the association reports.

Decorations, candles, cooking, and heating all pose heighten fire risk during the holidays. And today’s homes burn faster than ever, McKelvey cautions. A lot of modern homes are built with synthetic fibers and light-wood materials that burn fast and can produce toxic, vision-clouding gases.

“In a typical home fire, you may have as little as two minutes to escape safely from the time of smoke alarm sounds,” she says.

This narrow window of escape means once the smoke alarm goes off, it’s already too late to make a plan. You have to know what to do in case of fire before the fire ever starts, McKelvey stresses.

Preventing fires from starting is the best way to avoid harm.

So, why do Christmas trees burn so fast?

Christmas trees have unique properties that boost their burn rate. The trees’ large surface area and porous characteristic pulls in more oxygen that fuels fire.

Often, trees are positioned near curtains, presents, furniture, or rugs, which can quickly catch fire too, spreading fire quickly throughout a home. Burning furniture, like couches, can release toxic gases like polyurethane and carbon monoxide. Breathing in these gases can cause lasting respiratory damage and death, even if someone escapes a fire unscathed.

If you choose an artificial tree over a natural one, you’re not out of the woods.

“Artificial trees are made from plastics which have complicated burning behaviors, like melting and dripping,” Isaac Leventon, current researcher at the National Institutes of Standards and Technology said.

Many artificial trees are manufactured with polyvinyl chloride (PVC), a material that produces hydrochloric acid when it burns, Leventon explained. If purchasing an artificial tree, it’s important to choose a tree that’s flame retardant, if possible, McKelvey suggests.

All these factors combine to make Christmas trees potent fire hazards.

“Christmas trees are large, combustible items you’re bringing into your home,” McKelvey, the NFPA representative, reminds people.

They are more than holiday mainstays.

If the needles fall…

When it comes to Christmas trees, there are some effective strategies for preventing fire.

The first step in fire prevention happens before you ever bring your tree home, McKelvey says. When selecting a tree, choose wisely. Grab one of the branches and run your hand along it, the fire expert says. If needles fall into your hand, that the sign of the tree is drying out. You want to look for a well-hydrated tree where the needles don’t drop.

If you’re going the natural over artificial route, have the Christmas tree folks cut one to two inches from the base of the trunk before you take it home. Then, put the tree immediately in water once you bring it through the door. Fill your Christmas tree stand with fresh water every single day. Natural trees dry out over time, but daily watering can slow that process. The burn rate for a dry tree compared to well-hydrated tree is dramatic. Well-hydrated trees require a larger flame to ignite, while dry trees can go up in flames from a single spark.

"When selecting a tree, choose wisely.

Placement matters, McKelvey advises. Don’t place the tree blocking any windows or doors. In the case of fire, it’s critical to be able to access at least one exit in every room. Another good rule of thumb is to place the tree at least three to eight feet away from any heat sources— a fireplace or space heater, for example.

Light it up, carefully. Electrical distribution or lighting equipment was involved in more than two of every five (44 percent) home Christmas tree fires, with decorative lights the leading type of equipment involved, the National Fire Protection Association reports.

McKelvey advises following the manufacturer’s directions on Christmas tree lights as well as tossing out any lights with broken bulbs or frayed, loose wires. McKelvey stresses to never leave Christmas trees unattended for extended periods. Unplug the lights when you go to bed or leave the house.

A surprising tip? Be aware of how pets interact with the tree, too. Cats and dogs can chew on the wires and knock over trees, inadvertently heightening fire risk.

Lastly, don’t let the Christmas tree linger. The longer you have a tree in your home, the more combustible it becomes, McKelvey explains.

“Once the holidays are over it over, try and get it out of the house as soon as possible,” she says.

It’s unlikely your home will burn from putting up the iconic holiday decoration, but taking these forward-thinking steps will help ensure everyone can enjoy it in peace.