The adage that increasing outdoor air ventilation will improve IAQ is not always the case. For example, how do you manage ventilation to support occupant health if outdoor air is polluted by an upwind wildfire or an accident that spilled airborne toxins? Optimizing IAQ for occupant health is clearly essential during these episodically challenging events. Less well-studied is energy-efficient optimization of IAQ for health during times when outdoor pollutants are high from routine traffic or industrial emissions.
Outdoor pollution can be grouped into particulate matter and gases, with both potentially causing harm to health. Particles with diameters of 10 microns (PM10) or less irritate the upper respiratory track and eye membranes. Smaller particles with diameters of 2.5 microns or less (2.5 PM) are more harmful because they can be inhaled into deeper regions of the lungs, penetrating respiratory membranes, and travelling to distant tissues through the bloodstream. The inflammation associated with inhaling these small particles has been linked to increased disease outside the lungs, including heart attacks, strokes, and even dementia. The other general category of airborne pollutants are gases, which also can diffuse into the circulation and human tissues to cause disease in both the lungs and other organs. For example, odorless carbon monoxide enters the circulation through inhalation, binding to human red blood cells and displacing life-sustaining oxygen. The potentially lethal effect is caused by the lack of oxygen in the brain and other organs.
Outdoor air pollutants gain access to indoor environments through mechanical ventilation, natural ventilation, and passive infiltration. When should outdoor air be passively introduced into a building and when should resources be spent on conditioning outdoor air? When should the focus be on mechanical ventilation, upgrading filter efficiency or changing filters frequently, adding humidification or dehumidification to maintain the healthy relative humidity of 40%–60%, or adding safe air-cleaning systems?
To avoid both increased disease from outdoor pollutants and wasteful energy expenditure from unnecessary or counter-productive actions, the first step should be to measure both IAQ and outdoor air conditions to determine which — if any — contaminants are present in both settings. This can be accomplished by using both indoor and outdoor air quality sensors.
For example, a study in 2017 (Martins and Carrilho da Graça, 2017) monitored indoor and outdoor PM2.5 to guide use of natural ventilation versus mechanical ventilation. When outdoor PM2.5 levels were low, windows were opened to passively introduce outdoor air for cooling or dilution of indoor contaminants. Conversely, during periods of high PM2.5, windows were closed and HVAC-driven recirculation was used. The study reported HVAC energy savings of 25%–80% when the outside conditions were favorable. When outdoor particle levels were above 12 micrograms per cubic meter, natural ventilation was decreased. However, the energy-saving potential was still between 20%–60%.
Deploying air quality sensing platforms to give visibility of indoor and outdoor conditions should be visible to both building managers and occupants. Unfortunately, these areas of measurements have been largely separate and not used concurrently for optimal IAQ management. Thankfully, affordable sensors are now available, allowing the gathering, analyzing, and implementation of real-time data to operate buildings for both occupant health and energy efficiency.
