End of Summer: What Makes Winter Unfriendly with Indoor Environments?

As summer fades and winter approaches, the shift in seasons brings more than just colder weather—it significantly alters indoor environmental quality (IEQ). Homes, offices, and schools experience changes in temperature, humidity, air quality, and lighting that can negatively affect health and comfort. Understanding these factors can help us prepare our indoor spaces for a healthier winter season.

Reduced Ventilation and Air Quality

During the colder months, we naturally close windows and doors to conserve heat, which dramatically reduces ventilation rates. According to the U.S. Environmental Protection Agency (EPA), indoor air can be 2 to 5 times more polluted than outdoor air, and this concentration often rises in winter due to limited airflow (EPA, 2023). Pollutants such as volatile organic compounds (VOCs), mold spores, and allergens accumulate, increasing the risk of respiratory issues and allergies.

Additionally, the use of combustion-based heating systems can release carbon monoxide and nitrogen dioxide. A World Health Organization (WHO) report notes that indoor air pollution from heating sources contributes to 3.2 million premature deaths annually worldwide (WHO, 2022).

Low Humidity Levels

Winter air is naturally dry, and indoor heating systems further reduce relative humidity. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends maintaining indoor humidity between 30% and 60% for optimal health (ASHRAE, 2017). Levels below this range can dry out mucous membranes, making us more susceptible to infections like colds and flu. A 2020 study published in Annual Review of Virology found that influenza viruses survive longer and spread more easily in low-humidity environments.

Thermal Discomfort

Thermal comfort is another major challenge. Uneven heating and poorly insulated buildings can lead to cold drafts, while overheating in small spaces can cause discomfort and drowsiness. Research from the Lawrence Berkeley National Laboratory shows that even a 2°C deviation from thermal comfort can decrease productivity by up to 10% (LBNL, 2015).

Lack of Natural Light

Shorter days in winter limit exposure to natural sunlight, impacting circadian rhythms and mental health. Reduced daylight is linked to Seasonal Affective Disorder (SAD), a type of depression affecting about 5% of adults in the U.S.annually (American Psychiatric Association, 2021). Poor lighting indoors can exacerbate this by disrupting sleep patterns and lowering mood.

Preparing for Healthier Indoor Environments

Mitigating winter’s impact on IEQ requires proactive measures: increasing mechanical ventilation, using humidifiers, improving insulation, and maximizing natural light exposure. Upgrading air filtration systems and using low-VOC materials can further protect occupants from pollutants.

As winter sets in, prioritizing indoor environmental quality isn’t just about comfort—it’s about safeguarding health and well-being when we spend the majority of our time indoors.

References

1. U.S. Environmental Protection Agency (EPA). (2023). Indoor Air Quality.

2. World Health Organization (WHO). (2022). Household Air Pollution and Health.

3. ASHRAE. (2017). Thermal Environmental Conditions for Human Occupancy (Standard 55-2017).

4. LBNL. (2015). Indoor Environmental Quality and Productivity. Lawrence Berkeley National Laboratory.

5. American Psychiatric Association. (2021). Seasonal Affective Disorder.