Managing indoor air quality (IAQ) to support health is one of the biggest public health opportunities of this century. As with most opportunities, however, there are also significant challenges. One of those challenges is changing the metric historically viewed as synonymous with good or bad IAQ: ventilation rates. Ventilation rates, per se, do not relate to human health thresholds. 

While many of us grew up hearing the phrase, “go outside and get some fresh air,” we now know that outside air is not always fresh. Changing this familiar assumption is critical if we really want to manage IAQ to support occupant health and eliminate unnecessary HVAC energy consumption. 

The presumption that high ventilation rates are synonymous with good IAQ, and conversely, low ventilation rates with poor IAQ, may be true in some settings, but this is far from a universal truth. Furthermore, ventilation rates do not give a consistent assessment of occupant indoor exposures, the actual cause of health problems. While flow rates provide a useful metric for the design engineer predicting energy use management, occupant health is not considered. 

The only air metrics that directly reflect the impact of IAQ on occupant health are contaminant concentrations within a given range of temperature and humidity. 

Ventilation reduces the levels of many indoor contaminants via dilution, but prescribing ventilation alone is not a solution creating healthy IAQ. For example, when indoor sources exist, the most effective step is to eradicate emissions. Another situation where ventilation is not effective is when indoor relative humidity (RH) is low and exhaled droplets shrink to become tiny and remain airborne through random, Brownian motion. 

Furthermore, when contaminants are present in outdoor air, increasing ventilation has the potential to make IAQ worse. Buildings that are downwind from auto emissions, industrial sites, and ground dust sources (think farms) can have poorer IAQ when ventilation rates are increased. 

Traditionally, ASHRAE management of IAQ has been to require minimum ventilation rates in liters per person or liters per square meter of space. A relatively new IAQ procedure, part of 62.1, is performance-based guidance based on indoor concentrations of contaminants, but remediation solutions are still based on flow rates with filtration added when necessary. 

The building industry around the world has no consensus on best practices for outside air ventilation, filtration, contaminant extraction, recirculation, and airflow for room pressurization. And yet, these interventions impact the first cost and operating costs of the infrastructure supporting such facilities. 

“The available data do not provide a sound basis for determining specific outdoor air ventilation rates that can be universally applicable in different public and residential buildings to protect against health risks. They cannot be used for regulative purposes, unless the required ventilation rates are related to actual exposures and are prescribed only when full advantage of other methods for controlling exposures has been taken.” 

What direction should we move in? At the present time, many people are unaware of the health significance of indoor contaminants that they cannot smell or feel through irritation of their respiratory tract. Affordable sensors revealing many of the indoor constituents that impact health now exist. Wide use of monitors displaying IAQ concentrations collected by indoor sensors is now possible. With visible displays of the data, building operators can show their commitment to providing good IAQ, thus advancing the public´s awareness.