Christmas trees help set the holiday mood with twinkling lights, shiny ornaments, and the scent of pine. Each year, Americans buy approximately 25-30 million live Christmas trees to decorate their homes for the holidays (1). About 85% of those trees will be pre-cut weeks ahead of time – in most cases prior to Thanksgiving. The trees are cut, baled, and packed into refrigerated trucks to be delivered to a tree seller near your home.

Unfortunately, along the way, moisture and the tight bundling of the trees supports an ideal environment for mold to grow. In many cases, once the tree gets into your home the mold on the tree begins reproducing, triggering an allergic reaction known as “Christmas tree syndrome.” About 7% of the general population with allergies suffers from Christmas tree allergy symptoms that include (2):

• sneezing
• coughing
• congestion
• runny nose
• itchy nose and throat
• itchy or watery eyes
• dry, scaly skin

Christmas trees and mold

In one study, researchers closely measured mold counts in a room that contained a live Christmas tree. During the first three days that the tree was indoors, the count of mold spores in the air remained at about 800 spores per cubic meter of air. Starting on the fourth day, however, the spore counts began rising and eventually reached 5,000 mold spores in the air per cubic meter within two weeks (2). Counts higher than 700 spores per cubic meter are an indication that mold growth is occurring in an environment.

In another study, researchers examining 23 samples of bark and pine needles from Christmas trees found 53 different kinds of mold. Many of the mold varieties found on the trees were those most likely to trigger allergies – including (3):

• Aspergillus. This type of mold is especially dangerous for individuals with compromised immune systems.
• Penicillium. The mold from which penicillin is made, it causes allergic reactions in some people.
• Cladosporium. A common mold that can cause skin infections and allergic symptoms.

Although there is no single set of state or federal standards regarding acceptable indoor mold spore levels, the World Health Organization considers indoor concentrations greater than 500 spores per cubic meter to be unacceptable. At the very least, the elevated mold spore levels associated with live Christmas trees means that families with allergies should consider keeping a live tree indoors no more than a few days, especially if there are signs of increased allergies, which might be from Christmas tree syndrome.

Pollen can also be a problem

Pine pollen is not a major issue with Christmas tree allergy symptoms, because evergreen plants pollinate in the spring, not in early winter. However, the trees come into contact with allergens such as ragweed pollen and may carry it into the house where it can distress those with ragweed allergies.

Ragweed is an invasive species of the Asteraceae family and Ambrosia genus of plants (4). Ragweed is not a single plant but a collection of between 17 and 20 different species in North America that typically have tall, skinny stems with green flower spikes that do not contain nectar and are not attractive to pollinators.

Instead, ragweed pollen distribution is facilitated by the wind. And ragweed plants are prodigious producers of pollen – one plant can release up to one billion grains into the atmosphere – which can then travel hundreds of miles, though most of the pollen remains close to the plants that produced it (5).

These plants are more prevalent in the Eastern and Midwestern U.S., particularly in rural areas. There, common sites for ragweed invasions are in fields and along roadsides and riverbanks. In urban areas, the plants often flourish in vacant lots since turfgrass and other cultivated species help prevent ragweed from taking hold.

Seeds from ragweed species can remain dormant in the ground for decades before finally germinating.

And ragweed is not the only member from this family of plants that can latch on to Christmas trees. Other members include:

• sage
• rabbit brush
• groundsel bush
• burweed marsh elder
• mugwort
• eupatorium

Another allergen that a Christmas tree can be the source of in your house is grass pollen, which can stick to the tree’s sap during the spring. Then, when the tree is harvested and brought indoors, the sap dries out and the trapped pollen particles are released into the air.

The takeaway

If you still cherish the idea of a live Christmas tree in your home for the holidays, take heart. Here are a few simple steps you can take to reduce the likelihood that you or someone in your home will suffer from Christmas tree syndrome:

• Clean and wipe the trunk of your tree thoroughly with water and bleach before you bring it into the house.
• Use a leaf blower to remove as many pollen grains as possible before bringing the tree indoors.
• Get rid of the tree sooner. Mold spores increase the longer the tree is around. Consider removing the tree from your home the day after Christmas.
• Consider purchasing a self-cut tree. By cutting your tree yourself you bypass the storage and transportation conditions that promote mold growth.
• Use a high-performance room air purifier in the same room as the tree.
• Personal air purifiers can also help relieve the uncomfortable consequences of an allergic reaction.

By taking a few simple precautions such as these, you can make your holiday season decorative, delightful and, best of all, allergy free. If all else fails, consider switching to an artificial tree. It won’t fill your living room with the nostalgic scent of pine, but as long as you rinse any dust off of it thoroughly and don’t store it in a damp or humid place where mold can grow, it won’t trigger Christmas tree allergy symptoms either.