During the holiday season, approximately 30 million live Christmas trees are purchased by Americans. Many families not only delight in the presence of a live tree in their homes but also savor the fresh aroma it imparts. This fragrance is a result of volatile organic compounds (VOCs), chemical substances about which limited information regarding emission levels and potential health effects is available.
To address these uncertainties, researchers conducted an experiment using a common Christmas tree variety, the Douglas fir. They confined the tree within a chamber and monitored the quantity and types of VOCs emitted over a 17-day period. Additionally, they explored whether these VOCs interacted with other indoor air components, leading to the formation of new compounds.
The outcomes of the study have been documented in the Indoor Environments journal. The familiar scent associated with Christmas trees is attributed to a group of VOCs known as monoterpenes, also present in air fresheners, candles, and certain personal care products. In outdoor environments, conifers, including most Christmas trees, release monoterpenes, impacting outdoor air quality. However, the extent of monoterpene release when a tree is cut down and placed indoors remains largely unknown.
Studies indicate that monoterpenes can react with ozone, a compound crucial in the upper atmosphere for shielding against the Sun. Ground-level ozone, generated through light-induced chemical reactions, can cause symptoms such as coughing and throat irritation. Ozone readily reacts with other airborne chemicals to create new compounds, prompting researchers to investigate the effects of ozone in the presence of an indoor Christmas tree.
The researchers replicated a home environment in their experiment, adorning the tree with typical holiday lighting and subjecting it to a simulated day-night cycle with bright lights. They measured the emitted chemicals using proton-transfer reaction mass spectrometry (PTR-MS), a technique capable of detecting airborne organic compounds, over the 17-day period.
Monoterpenes proved to be the most prevalent VOC emitted from the tree, peaking on the first day before significantly diminishing by the third day. Their concentration initially matched that of a plug-in air freshener or a newly constructed house but swiftly dropped to nearly one-tenth of the original amount. The researchers identified 52 distinct types of monoterpenes.
Introducing ozone into the chamber revealed its reaction with monoterpenes, resulting in byproducts such as formaldehyde, another VOC, and other reactive chemicals. The presence of ozone further reduced monoterpene concentration while elevating formaldehyde levels, indicating an impact on indoor air chemistry. However, the quantity of formaldehyde generated was relatively small, around 1 part per billion, compared to the typical 20 to 30 parts per billion in U.S. households.
For individuals sensitive to VOCs, Christmas trees might be a potential cause of watery eyes and noses, especially upon initial placement indoors. In such cases, opening a window near the tree can reduce exposure. Additionally, allowing newly cut trees to remain outdoors or in a garage for three days before bringing them indoors can mitigate exposure, as the emission strength naturally decreases over time.