How do fresh air, cold air, and fresh cold air affect upper respiratory tract infections?

Effects of Fresh Air, Cold Air, and Fresh Cold Air on Upper Respiratory Illness

Fresh air through adequate ventilation is one of the most effective measures to reduce the spread of upper respiratory infections, while cold air exposure can trigger respiratory symptoms and increase susceptibility to infection through mucosal drying and impaired immune defenses. 1

Fresh Air and Ventilation: Protective Effects

Good ventilation with fresh outdoor air is beneficial in preventing infectious disease transmission by diluting airborne pathogens and reducing indoor air pollutant concentrations. 1

• The WHO 2007 guideline identified adequate natural ventilation as one of the most effective measures to reduce the spread of infection, even in non-healthcare settings like homes. 1
• A review of 360 studies investigating housing risk factors for respiratory disease reported that poor indoor air quality, mainly due to lack of adequate ventilation, was the most influential risk factor for respiratory illness. 1
• Indoor air pollutant levels are typically 2-5 times higher than outdoor levels, and in some cases can exceed outdoor levels by 100 times, making fresh air dilution critical. 1
• Ventilation reduces exposure to human bioeffluents, pathogens (including virus droplets), and indoor pollutants, thereby reducing both acute and chronic respiratory health outcomes. 2

However, a critical caveat exists: Ventilation with dry outdoor air (low absolute humidity) can paradoxically increase respiratory vulnerability and infection risk despite diluting pathogens. 3, 2

Cold Air: Detrimental Effects on Respiratory Health

Cold air exposure directly triggers respiratory symptoms and increases susceptibility to upper respiratory infections through multiple mechanisms. 1

Immediate Physiological Effects

• Inhalation of cold, dry air (CDA) causes drying of the nasal mucosa and creates a hyperosmolar environment that activates mast cells, leading to inflammatory mediator release and subsequent symptoms including rhinorrhea. 1
• CDA exposure produces both an immediate inflammatory response and a late-phase reaction lasting up to 10 hours, with increased histamine levels and markers of vascular permeability. 1
• Epithelial cells are found in increased numbers in nasal lavage after CDA exposure, suggesting disruption of tight junctions in the protective epithelial barrier. 1

Increased Infection Risk

• Low temperature may favor virus viability and increase the risk of cardiovascular and respiratory diseases. 2
• Cold housing conditions have strong research evidence linking them to increased respiratory and cardiovascular effects. 1

Fresh Cold Air: The Humidity Factor

The combination of fresh but cold air presents a complex scenario where the benefits of ventilation must be weighed against the risks of low humidity and cold temperature. 3, 2

The Critical Role of Humidity

• Low indoor air humidity (below 40% relative humidity) increases acute eye and airway symptoms, reduces mucociliary clearance, weakens immune defense, and deteriorates work productivity. 3
• Dry air causes vulnerable eyes and airways from desiccation and less efficient mucociliary clearance, elevating mucous membrane-related symptoms. 2
• Low relative humidity increases the generation of infectious virus-laden aerosols exhaled from infected people and increases viability of influenza and COVID-19 viruses. 3
• The optimal "Goldilocks zone" for minimizing infection risk is 40-60% relative humidity. 3, 2

Practical Implications

• Ventilation with cold outdoor air during winter months typically brings very dry air (low absolute humidity) indoors, which when heated becomes even drier in relative terms. 3
• The negative effects of ventilation with dry outdoor air should be assessed against: 1) weakened health and functionality of airways, 2) increased viability and transmissibility of infectious viruses, and 3) increased floating time of virus-containing droplets in indoor air. 3

Environmental Triggers and Pollutants

Outdoor air pollutants can trigger or exacerbate respiratory symptoms, particularly in susceptible individuals. 1

• Air pollutants such as nitrogen oxides from outdoor traffic have been linked to increased respiratory symptoms including cough and increased symptoms from respiratory viral infections, particularly in children. 1
• Outdoor air pollutants such as ozone can cause cough and increase airway inflammation, triggering asthma symptoms, with greater effects in those with preexisting poorly controlled asthma. 1
• Natural ventilation or mechanical ventilation can become sources of outdoor allergens and pathogens, potentially increasing exposure rather than reducing it. 2

Clinical Recommendations

The optimal strategy is to maintain adequate ventilation while controlling indoor temperature (22-24°C) and relative humidity (40-60%). 2

• Elevation of indoor air humidity from dry air conditions improves health of airways, reduces acute symptoms, improves work performance, and reduces risk of infection. 3, 2
• The "health-based ventilation rate" should meet WHO air quality guidelines while being integrated with both indoor air humidity and room temperature in strategic joint control. 2
• Building to higher insulation standards is effective in reducing respiratory and cardiovascular effects of cold housing. 1

Common pitfall to avoid: Do not assume that simply increasing ventilation with outdoor air is always beneficial—during cold, dry weather conditions, uncontrolled ventilation can worsen respiratory health by introducing excessively dry air that compromises mucosal defenses. 3, 2