How Christmas trees produce that fresh, festive scent

Each holiday season, nearly 30 million American households celebrate with a live Christmas tree, embracing not only the visual splendor but also the delightful scent that permeates their homes.

This scent, evocative of the holiday spirit, originates from volatile organic compounds (VOCs). Despite their ubiquity during the season, the extent of these emissions and their potential health impacts remain largely uncharted.

Christmas tree scent and VOCs

Dustin Poppendieck, an environmental engineer at the National Institute of Standards and Technology (NIST), emphasizes the significance of understanding these emissions.

“Our nose is a good chemical sensor,” said Poppendieck. “We know that these trees are emitting something, and the question then becomes: How big of a source is it? We wanted to explore which chemicals are emitted and how much, and to put that into the context of other sources of chemicals in a house,” he said. 

Poppendieck and his team embarked on a journey to decode the type and quantity of chemicals emitted by Christmas trees, seeking to contextualize this within the broader spectrum of household chemical sources.

Capturing the essence of Christmas Trees

The NIST researchers selected a Douglas fir, a popular Christmas tree choice, for their study. They enclosed it in a sealed chamber to accurately measure the type and quantity of VOCs released over 17 days.

A critical part of their investigation included understanding how these VOCs interacted with other indoor air components, potentially forming new compounds.

Monoterpenes, a group of VOCs, are the primary contributors to the Christmas tree’s signature smell. These compounds are also present in various everyday items, like air fresheners and personal care products.

Although their effects on outdoor air quality, due to emissions from conifers, are known, the impact of indoor emissions, especially from freshly cut trees, is less understood.

Christmas trees, ozone, and scent

The interaction between monoterpenes and ozone, a ground-level pollutant known for causing respiratory issues, was a focal point of the study.

The researchers simulated a typical home environment, complete with holiday lights and a day-night cycle, in their controlled chamber.

Using advanced techniques, they monitored the tree’s VOC emissions, particularly focusing on the effects of introducing ozone into the environment.

The most abundant VOC emitted was monoterpenes, peaking on the first day and significantly diminishing thereafter.

Results of the study

Initially, their concentration was on par with a plug-in air freshener or a newly constructed house, but it rapidly decreased to nearly a tenth of its original level. In total, 52 distinct types of monoterpenes were identified.

When ozone was introduced, it reacted with the monoterpenes, forming byproducts like formaldehyde and other reactive chemicals. While the monoterpene concentration decreased further, formaldehyde levels increased.

However, the formaldehyde concentration remained relatively low, at around 1 part per billion, compared to typical household levels of 20-30 parts per billion.

Health implications and precautions

For individuals sensitive to VOCs, the initial period after bringing a Christmas tree indoors could cause mild symptoms like watery eyes or a runny nose.

To mitigate this, Poppendieck suggests keeping a window open near the tree or leaving the tree outside or in a garage for three days before bringing it indoors, as VOC emissions naturally decline over time.

Nevertheless, Poppendieck reassures, “For most people, this shouldn’t be a major concern. I’m still planning to have a Christmas tree in my house.”

Water your Christmas tree to keep the scent

Apart from air quality considerations, it’s vital to water your Christmas tree daily. The most significant risk stems from a dried-out tree, which can pose a serious fire hazard in your home.

Through this study, we gain a deeper understanding of the air quality dynamics associated with live Christmas trees, empowering us to make informed decisions while still enjoying this cherished holiday tradition.

NIST has safety tips available here.

The full study was published in the journal Indoor Environments.

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