Allergies at home in winter

ACAAI says indoor allergies are a year-round problem but tend to be noticed more in winter, because people spend more time at home, are likely to keep doors and windows shut against the cold, and run heating systems that recirculate the indoor air and any allergy triggers in it.¹ Outdoors, the picture is quieter: the Texas A&M Forest Service says very few plants pollinate during the winter, with juniper trees as the exception.¹⁴

Why a closed-up house matters

EPA notes research indicating that people spend approximately 90 percent of their time indoors.² It says inadequate ventilation can increase indoor pollutant levels by not bringing in enough outdoor air to dilute emissions from indoor sources and by not carrying pollutants out.² Storm windows, weather stripping, caulking and blown-in wall insulation can reduce the outdoor air that seeps into a home, EPA says, and concentrations of pollutants from sources inside the home can then increase.²

EPA says some biological contaminants trigger allergic reactions, including allergic rhinitis and some types of asthma.² ACAAI points out that pets spend more time indoors in winter too, which increases exposure to pet dander and saliva.¹

A University of Kentucky entomology fact sheet says house dust mites and their allergenic particles are present in homes year-round, but people tend to have fewer symptoms during the summer, perhaps because they spend more time outdoors.¹¹

Dust mites in a heated home

AAAAI says dust mites thrive in warm, humid environments such as bedding, upholstered furniture and carpeting.⁸ EPA says their body parts and droppings can trigger asthma in people allergic to them.³

A practice parameter from US allergy societies says dust mites tend to be found where there is warmth and moisture, and that they can survive in cold, dry climates by living in homes that are artificially heated.¹⁰ It says dust mite allergen levels tend to rise during the summer, when humidity is high, and stay elevated through the winter before decreasing during the late winter and spring.¹⁰ Once humidity drops below 50%, the same document says, mites multiply less and survive less well.¹⁰

Dry winter air brings a trade-off: AAAAI says dry indoor air in winter is often the cause of chapped lips, dry skin and irritated sinus passages, but that moisturizing the air with a humidifier creates the perfect home for dust mites.⁹ For people with indoor allergies, it says, dust and mold from a humidifier may cause more harm than good.⁹

Mold and damp spots indoors

EPA’s mold guide says there are many types of mold, and none of them will grow without water or moisture.⁴ AAAAI says indoor molds and mildew need dampness of the kind typically found in basements, bathrooms or anywhere with leaks.⁸ The CDC says mold can get into a home through open doors, windows and vents and through heating and air conditioning systems, and that the most common indoor molds are Cladosporium, Penicillium and Aspergillus.⁷

EPA’s guide also says condensation can be a sign of high humidity, and it names windows, walls and pipes as places where condensation or moisture can collect.⁴ It calls allergic reactions to mold common and lists hay fever-type symptoms such as sneezing, runny nose, red eyes and skin rash.⁴ Molds can also cause asthma attacks in people with asthma who are allergic to mold, it says.⁴

ACAAI suggests wiping off holiday decorations during the holidays, especially ones stored in areas where dust and mold proliferate.¹ Our guide to mold spore counts covers outdoor mold and how counting stations measure it.

Humidity ranges from US agencies and allergy groups

The sources below give overlapping humidity ranges or ceilings for homes, each for its own purpose.

Source	Range it gives	Context
EPA, asthma triggers page	Low, ideally between 30 and 50% relative humidity	Molds
EPA, mold and moisture guide	Below 60% if possible, ideally between 30 and 50% relative humidity	Mold
EPA, indoor air quality guide	30 to 50%, “generally recommended for homes”	Limiting biological contaminants
CDC, mold page	“As low as you can,” no higher than 50%, all day long	Mold
AAAAI, humidifier page	Between 40 and 50%	Humidifiers and dust mites
Dust mite practice parameter	35% to 50%	Decreasing dust mite growth

Each row restates the source it names.^3427910 Our dehumidifier finder compares the certified models on EPA’s list.

Pets, cockroaches and mice in the cold months

EPA says proteins in a pet’s skin flakes, urine, feces, saliva and hair can trigger asthma.³ AAAAI says there are no “hypoallergenic” breeds of dogs or cats.⁸

A practice parameter on cockroach exposure says cockroaches tend to be found in southern latitudes because they need both warmth and moisture, and are usually not found in northern latitudes unless they can live in places that people have artificially warmed.¹² It says single-family homes in colder climates have a lower risk of infestation.¹² EPA says droppings or body parts of cockroaches and other pests can trigger asthma.³

A University of Wisconsin wildlife outreach specialist says that with diminishing daylight and cooler temperatures, some warm-blooded rodents try to come into warm homes for shelter and, in some cases, for food.¹³ AAAAI says urine is the source of allergens from mice, as it is for rabbits, hamsters and guinea pigs.⁸

Fireplaces, wood stoves and fuel-burning appliances

EPA says smoke from wood-burning stoves and fireplaces contains a mixture of harmful gases and small particles, and that breathing these particles can cause asthma attacks and severe bronchitis.³ Its wood smoke page calls fine particles, or PM2.5, the biggest health threat from smoke, and says they can make asthma symptoms worse and trigger asthma attacks.⁵ It also says wood smoke can irritate the lungs, cause inflammation and affect the immune system, and that in wintertime residential wood smoke is a main contributor to fine particle pollution and is responsible for poor air quality days in many areas across the United States.⁵

The American Lung Association says fireplaces and woodstoves can create harmful wood-burning emissions indoors, and that indoor air can also be affected when neighbors burn wood.⁶ It says emissions from wood smoke can cause coughing, wheezing and asthma attacks, among other harms.⁶ EPA’s indoor air guide adds that pollutants from fireplaces and woodstoves with no dedicated outdoor air supply can be back-drafted from the chimney into the living space, particularly in weatherized homes.²

EPA says indoor nitrogen dioxide can come from appliances that burn fuels such as gas, kerosene and wood, and that the gas can irritate the eyes, nose and throat and cause shortness of breath.³

Outdoor pollen and the cedar exception

The Texas A&M Forest Service says most trees pollinate in the spring and that ragweed pollen and mold spores can contribute to allergies in the fall, but very few plants pollinate during the winter.¹⁴ Juniper trees are the exception: the Forest Service says they typically begin producing pollen in mid-December, reach a peak in mid-January and then slowly taper off toward the beginning of March.¹⁴ It describes cedar fever as an allergic reaction to the pollen released by mountain cedar trees.¹⁴ Mountain cedar is one of the common names of Ashe juniper, whose main population grows in west-central Texas, largely on the Edwards Plateau, with separate populations in southwestern Missouri and Arkansas, southern Oklahoma and Coahuila, Mexico.¹⁵ Cedar fever explained covers the tree, its season and why its pollen travels so far.

Because mountain cedar pollinates in the middle of cold and flu season, the Forest Service says it is not uncommon for people with cedar fever to mistake their symptoms for a cold or the seasonal flu.¹⁴ Winter allergies or a cold? compares the two.

Finding the cause

ACAAI says that after a consultation, an allergist may perform tests to diagnose allergies and identify potential causes.¹ AAAAI suggests that people with indoor allergies check with an allergist about whether a humidifier could help them.⁹ For questions about allergies at home in winter, talk to an allergist.