We don’t yet have effective sensors to measure all possible pollution-control inputs. Ideally, ventilation controls should control all hazardous contaminants affecting indoor air quality (IAQ). These control variables include contaminants, odors, CO2, temperature, humidity, and occupancy. Future sensor and control development may someday make this IAQ optimization possible.
Pollutants that Ventilation Can and Can’t Control
Ventilation isn’t a perfect solution for ensuring good IAQ but only a method to dilute remaining pollutants after you minimize pollutants at their source.
Therefore, we need to separate pollutants into three categories based on whether ventilation is effective at reducing them or not.
- Pollutants that respond well to control by ventilation, such as water vapor, VOCs, and carbon dioxide. Although water vapor isn’t itself a pollutant, water-vapor condensation provides sustenance to mold, mildew, and dust mites that produce respirable bioaerosols. Carbon dioxide isn’t an important pollutant, but it’s concentration correlates with the concentration of other occupant-related pollutants. Therefore ventilation specialists use CO2 to predict the concentration of more important occupant-related pollutants.
- Pollutants that don’t respond well to ventilation such as radon, carbon monoxide, and smoke from tobacco and candles. The concentration of these pollutants depends more on occupant behavior and home characteristics than on ventilation performance. Instead, these pollutants require effective source control.
- Polluted outdoor air that infiltrates the building bringing fine particles, ozone, and other contaminants.
Different sizes of particles create different risks of respiratory problems. LBNL researchers are most concerned about particles between one and three microns (µm) in diameter, which are abbreviated PM2.5. These particles are more likely than larger particles to become lodged deep in the lungs.
Scientists now agree that particles must not control ventilation airflow. Research indicates that indoor-outdoor air exchange from ventilation or open windows can even increase the particle count of the indoor air.
Complementary air filters are far more effective at particle control than ventilation systems that merely exchange indoor air with outdoor air. Ventilation systems, the forced air HVAC systems, or standalone air cleaners may provide these air filters. Air filters must have a MERV rating of 13 or above in order to capture a majority of the PM2.5 particles. Filters with MERV ratings above 13 may reduce airflow, increase energy consumption, and decrease the efficiency of forced-air HVAC systems, unless those systems are designed to handle the high static pressure that these filters create.
Volitile Organic Compounds (VOCs)
Research hasn’t established whether individual VOCs are effective as controlling IAQ variables for ventilation systems — either for DCV or smart ventilation. Our knowledge about VOCs isn’t yet comprehensive enough to fully understand their IAQ risk because of the following issues.
- Human sensitivity to VOCs is largely unknown except for formaldehyde and acrolein.
- VOC concentrations in nonindustrial buildings are often well below established thresholds.
- Susceptibility to VOC problems depends on the individual.
- The composition of VOCs varies from building to building.
Scientists instead recommend minimizing the VOC-emission rates of building materials and furnishings in new and renovated buildings. VOC emissions from building materials and furnishings increased since the 1990s but may now be decreasing because of State and federal regulation. Ventilation specialists should consider VOC sensors along with CO2 sensors where VOC levels are likely to remain high for long periods. Researchers have identified more than 300 different VOCs in indoor air, which makes measuring them all challenging.
Scientists refer to the total VOC concentration (TVOC) as possible controlling variable for ventilation systems. However the TVOC currently lacks a precise definition or a standardized procedure for its calculation. As yet there is no consensus for whether VOCs are an appropriate controlling variable for DCV and smart ventilation.