During 2012-2013, my research group in collaboration with the Fierer research group at the University of Colorado Boulder conducted a study of the microbiome and indoor air quality in 15 Front Range area homes. This study was funded by the Alfred P. Sloan Foundation. We examined many factors, of which two indoor air quality parameters were CO₂ and indoor particulate matter concentrations over eight separate 24-hour time periods during the course of a year. Stay tuned, this research will be published this year!

One home in particular had extremely high particulate levels; this home had been weatherized and was heated only with radiant heating from flooring. It was also cooled by a swamp cooler in the summer. A blower door test yielded a leakage rate of 3.88 air changes per hour at 50 Pa above the ambient pressure (ACH₅₀); buildings with values below 5 ACH₅₀ are considered very tightly sealed. Although this design is excellent in terms of keeping energy costs low, when used in conjunction with radiant heating the low air exchange rates may create health problems due to the potential buildup of indoor air pollutants within the home. The homeowners had described to our research team that they were often sick after moving into this home with asthma symptoms and respiratory infections, and even began using asthma inhalers during the winter months. At the conclusion of our home study we suggested the family install a new ventilation system in their home in order to increase their air exchange rates.

Recently the family installed an Energy Recovery Ventilation (ERV) system. This system is designed to always bring some outside air into the home. Since the installation the homeowners noticed their health has significantly improved and they were able to stop using their inhalers. At the request of the homeowners, a follow up assessment was performed on the home in March 2015 to determine the impact of the ERV system on the indoor air quality in the home. The TSI DustTrak that was used in the previous study was no longer available for the follow-up study therefore a Dylos Pro1100 was used to measure particulate matter counts. The PM2.5 counts from the Dylos were then converted into mass concentrations using the calibration curve determined by Klepis et al [1]. The PM10 counts were converted into mass concentrations by using the log mean diameter to determine average particle volume and assuming a density of 2 g/cm³; this method is not ideal for this application due to the potential for errors from density and volume assumptions. CO₂ measurements were recorded using the same TSI Q-Trak from the previous study. The instruments were setup in the same locations as the 2013 study, and CO₂ concentrations and particulate counts were recorded each minute over the course of 5 days.

Figure 1: Daily averages of PM2.5

Figure 1 and 2 represent the daily averages for PM2.5 and PM10 respectively. The results from the data collected during the 2013 study are shown in blue and the results from the 2015 reevaluation study are shown in magenta. ‘Mostly unoccupied’ indicates days which the home was unoccupied for more than 20 hours. The continuous PM2.5 and PM10 levels for all five days of the reevaluation are shown in Figure 3. The large peaks seen in the PM2.5 and PM10 on April 11^th, 2013 were due to a cooking incident where food was burned in the kitchen. The average PM2.5 concentration for the 2015 study was 4.7 µg/m³. This is much lower than the average PM2.5 concentration of 18.7 µg/m³ found in the 2013 study. The PM10 concentrations cannot be reliably compared due to potential conversion errors, but by observing relative magnitudes in Figure 2, it is clear the PM10 concentrations have been greatly reduced.

Although there are no existing regulations for indoor airborne particulate matter, the National Ambient Air Quality Standards (NAAQS) set by the EPA currently require the 24-h average ambient PM2.5 levels to be below 35 µg/m³. Elevated PM2.5 levels have been attributed to respiratory and cardiovascular illness as well as aggravating existing conditions. The high particulate matter levels in the home were mostly related to cooking activities, based on time-activity diaries. This home also has a gas stove and hood installed over the stove, but it did not seem to be very effective. Nitrogen oxides were not measured in this study but could have been elevated during cooking events.

Figure 2: Daily averages of PM10

In summary for this home that was weatherized and unvented, the residents experienced adverse respiratory symptoms while living in this home. Upon installation of the new ERV system, the residents reported much better health and our preliminary data shows a reduction in particulate matter levels.

CO₂ levels are used to understand the amount of fresh air the HVAC is bringing into the building, versus simply recirculating the air. The current health safety standard set by ASHRAE is 5000 ppm, and buildings with time-averaged value above 1000 ppm can be considered inadequately ventilated. The CO₂ levels in the home from 2013 were 629 ppm, and after the installation of the ERV system the CO₂ levels were reduced to 614 ppm. Figure 4 presents the CO₂ concentrations, with blue representing the 2015 daily averages and magenta representing the 2013 averages. Both the pre and post ERV CO₂ levels were well under the recommended CO₂ limits, but the reduction in CO₂ with the new system is encouraging because it indicates an increase in outside air entering the home.

Figure 3: Continuous particulate matter concentrations

Figure 4: Daily CO2 averages

1. Klepeis NE, Hughes SC, Edwards RD, Allen T, Johnson M, et al. (2013) Promoting Smoke-Free Homes: A Novel Behavioral Intervention Using Real-Time Audio-Visual Feedback on Airborne Particle Levels. PLoS ONE 8(8): e73251. doi:10.1371/journal.pone.0073251