There are between 25 million and 30 million live Christmas trees in the U.S. every year. Surprisingly, to the best of our knowledge, the indoor emissions of volatile organic compounds (VOCs) from these trees have yet to be studied.
When a typical Christmas tree is cut down and brought indoors, there is a big gap in our understanding of how volatile organic compound (VOC) emissions change.
These trees are known to emit some chemicals, and the question is: how big is it?
To solve these problems, NIST scientists used a common Christmas tree, Douglas fir, indoors. Over a 17-day period, they measured the amount and type of volatile organic compounds it released. In addition, they explored whether these VOCs react with other indoor air components to form new combinations.
The familiar fresh smell associated with Christmas trees comes from volatile organic compounds (VOCs) known as monoterpenes. These compounds are also found in air fresheners, candles, and personal care products.
Outdoor and conifer trees, including most Christmas trees, release monoterpenes that affect outdoor air quality. However, little is known about the amount of monoterpenes released when trees are cut down and brought indoors.
Studies have shown that monoterpenes can react with ozone. While ozone in the upper atmosphere is a protective layer against the sun, on the ground, ozone is formed through a chemical reaction with light, causing symptoms such as coughing and throat irritation. Ozone also reacts easily with other airborne chemicals to form new compounds.
The scientists were curious about the effects of ozone when indoor trees were present, so they placed it in a controlled room. They measured the volatile organic compounds (VOCs) emitted over a 17-day period using proton transfer reaction mass spectrometry (PTR-MS), a method for detecting organic compounds in the air.
In their experiments, the scientists recreated a home environment. They decorate the tree with typical holiday lights, exposing it to a simulated day-night cycle with bright lights, turning off the lights every 12 hours and watering the tree daily. They introduce outdoor air at a rate common in homes and continuously monitor indoor air for chemicals.
Monoterpenes are the most abundant volatile organic compounds emitted from trees. Their concentrations peak on the first day and decrease significantly by the third day. Initially, the concentration was comparable to plug-in air fresheners or newly built houses, but later dropped by a factor of almost ten. The researchers identified 52 different types of monoterpenes.
After introducing ozone indoors, scientists observed that ozone reacted with monoterpenes, resulting in byproducts such as formaldehyde, another VOC, and other reactive chemicals. With the addition of ozone, the concentration of monoterpenes decreases further, while formaldehyde levels increase, affecting indoor air quality. However, the formaldehyde produced is relatively small, about one part per billion. The formaldehyde concentration in a typical U.S. house is between 20 and 30 parts per billion.
Dustin, an environmental engineer at the National Institute of Standards and Technology (NIST), said, "For people who are sensitive to volatile organic compounds, Christmas trees can be a possible cause of watery eyes and nose, especially when initially brought indoors." In this case, opening a window near the tree can reduce inhalation. In addition, newly cut trees can be kept outdoors or in a garage for three days before being brought home, as emission intensity naturally decays over time. ”