Each year, around this season, a debate resurfacing is the most common holiday decoration: the Christmas tree. Should it be natural or artificial? Both options carry strong arguments. From an environmental angle, a natural tree can be preferable, even if it might seem counterintuitive because it often emits less CO2 during production than an artificial tree. From a health perspective, live trees can influence indoor air chemistry and may affect people with allergies or sensitivities to volatile organic compounds (VOCs).
A recent study examined the effects of natural Christmas trees on home air quality and the well-being of residents. Many families opt for live trees for their fresh fragrance, yet that scent comes from VOCs—chemicals present in the air. Until now, the exact emission levels and health implications of each tree were not well understood.
“Our nose is a handy chemical sensor,” explains Dustin Poppendieck, an environmental engineer at the National Institute of Standards and Technology (NIST). “We knew these trees were emitting something, and the goal was to identify which chemicals are released, in what amounts, and how they fit into the overall chemical load in homes,” he notes.
Planting a fir tree for Christmas. Pixabay
To resolve these questions, Poppendieck and colleagues sealed a spruce tree in a controlled room and tracked the VOCs emitted over 17 days. They also investigated whether these VOCs could react with other indoor air components to form new compounds.
Cough and throat irritation
The researchers’ findings, published in Indoor Environments, show that the familiar Christmas-tree scent originates from a group of VOCs called monoterpenes. These compounds are also found in scented products, air fresheners, candles, and some personal care items.
Outdoors, conifers— the group that includes most Christmas trees—release monoterpenes and can influence outdoor air quality. Yet the indoor release levels when a tree is cut and placed inside a home were less well known.
Prior studies indicate that monoterpenes can react with ozone, formed when pollutants react with light, potentially causing symptoms such as coughing and throat irritation. Ozone can also react with other indoor chemicals to generate new compounds. The team aimed to observe these dynamics in a simulated home environment with an indoor tree.
One of the researchers beside the experimental tree. M. King/NIST
In a environmentally controlled chamber, the tree’s emissions were measured in real time using PTR-MS, a technique that detects trace organic compounds in the air. The simulated home atmosphere was decorated with a typical Christmas lighting setup, with a day-night cycle, and the tree was watered daily while outdoor air was introduced at a rate typical for homes. Continuous air monitoring captured the evolving chemical profile inside the chamber.
Reactive chemicals
Monoterpenes emerged as the most abundant VOCs, with 52 distinct compounds identified. Concentrations peaked on day one, then dropped sharply by day three. Initial levels were comparable to a plug-in air freshener or a newly built house, but over time they decreased to roughly a tenth of the original amount, according to Poppendieck.
The scientists then introduced ozone into the chamber to study its impact on indoor air chemistry. They found that ozone reacts with monoterpenes to form byproducts such as formaldehyde and other VOCs and reactive chemicals.
With ozone present, monoterpene levels further declined while formaldehyde levels rose, altering indoor air chemistry. However, the formaldehyde concentration remained very low, around one part per billion, far lower than typical indoor levels in many homes.
For individuals sensitive to VOCs, a live Christmas tree could cause watery eyes or a runny nose during the first days of indoor use. Poppendieck suggests practical measures: open a window near the tree to dilute VOC exposure. If possible, outdoor storage or keeping a freshly cut tree in the garage for a few days can reduce emission intensity before bringing it inside.
Overall, the researcher notes that, for most people, placing a freshly cut natural tree indoors should not pose a major concern, and many still choose live trees for their aroma and tradition.
New method: trees for rent
When considering environmental impact, a seemingly greener option might be an artificial tree, but plastics like PVC used in many artificial trees can generate greenhouse gases during production and are not biodegradable. Independent estimates suggest that a two-meter artificial tree can emit about 40 kilograms of CO2, while a natural tree of similar size emits roughly 3.5 kilograms. To surpass the environmental advantage of natural trees, an artificial tree would need to be reused about a dozen times.
Proponents of fake trees highlight cost savings through reuse, a lack of pollutants such as fungi, and a pine-like aroma. In contrast, supporters of live trees emphasize local sourcing, supporting local economies, and sustainability, noting that a large share of artificial trees are manufactured abroad, with a sizable portion coming from China.
In the United Kingdom, some outfits are exploring tree rental programs. These trees are grown in special pots that allow them to be dug up and replanted each year. Customers pay a deposit and receive guidance on maintaining tree health, such as avoiding heat sources and watering regularly.
Reference report: ScienceDirect article S2950362023000024
…
Environmental department contact: [email protected]