This study shows that a school with mechanical ventilation and a variable volume make-up air system using MERV 8 pleated filters has significant reductions in small particles but lower than expected reductions in particles in the 3 to 10 micrometer range.
Health professionals often recommend that asthmatics and others with respiratory diseases stay indoors during periods of high outdoor air pollution. Components of pollution such as Ozone, various Nitrogen compounds and particulate matter have all been shown to have negative effects on these individuals. However, there is the potential for outdoor to indoor transfer and the question arises as to just how beneficial “staying indoors” really is. What protection does an indoor environment afford?
Most children spend a significant portion of their time in school buildings. To what extent does the typical school environment lessen exposure to outdoor air pollution? A recent study conducted by researchers from the University of Utah gives us some information on this question. In an article published in the Journal Indoor Air, Parker et al. analyze indoor particle counts in a school and compare them to outdoor particle counts in the playground of the same school. The school has a variable air volume HVAC system with return air and fresh air intake . The make up air is controlled by a CO2 monitor set at 800-1,000 ppm. The study took place at a school in Utah in January and February so only the heating portion of the system was used. MERV 8 filters were used in the HVAC units. The system was designed so that both the return air and the outside air pass through the filters.
MERV 8 filters have an average particle size efficiency of 70% or more in the particle sizes between 3 and 10 micrometers (E3). They are not assessed on their ability to collect particles in the 0.3 to 1 micrometer range (E1) or the 1 to 3 micrometer range (E2). This does not mean that MERV 8 filters do not capture particles in these ranges. It only means that the filter efficiency levels in these smaller groups are not used to calculate the final MERV number for a filter. It is important to understand that all filters capture at least some particles in all size ranges. It is only those filters with a MERV 9 and above that are assessed on efficiency by particle size in the 1 to 3 micrometer range and MERV 13 that are assessed on their efficiency on particles in the 0.3 to 1 micrometer range.
Ratio of Indoor Particles to Outdoor Particles
| Size | Occupied | Vacant |
| 0.5 um | 0.1 | 0.09 |
| 1 um | 0.16 | 0.11 |
| 2.5 um | 0.35 | 0.03 |
| 5 um | 1.26 | 0.07 |
| 7.5-10 um | 11.3 | 0.2 |
Particle counts were taken on five of the most polluted days and five of the least polluted days. It is very interesting to see the particle counts in this school as they compare to outside particle counts in all particle size ranges. They are very informative in that they show the effects of the protection of the building envelope and the HVAC system filter efficiency.
Measurements of particle mass at PM2.5 showed that indoor mass was 10% of outdoor mass when the building was “occupied.” When the building was “not occupied” the indoor particle mass at PM2.5 was 6% of the outdoor particle mass. In the 6 size ranges from 0.3 to 1.6 micrometers both the “occupied” readings and the “non-occupied” readings were about 10% of outdoor particle counts. But starting at the 1.6 micrometer range the “occupied” readings start to exceed the “non-occupied” readings. At 1.6 micrometers the “occupied” count is 35% of the outdoor count while the “non-occupied” count is 15% of the outdoor count. On particles between 4 and 5 micrometers the “occupied” count is 52% of the outdoor count while the count in this size range when the building is vacant is 2% of the outdoor count. When particles go up to the 7.5-10 micrometer range the indoor “occupied” count is actually 11.3 times (1130%) the outdoor count while the vacant building count is 20% of the outdoor count.
While the combination of the building envelope and the HVAC system do a good job of reducing particles in the smaller size ranges, the movement of the many people in the school add significantly to the larger particle counts. Many allergens are 5 micrometers and above and it is likely that the resuspension of larger particles could lead to higher allergic reactions in a school environment. Other studies have shown that cat dander is present on the clothes and hair of students who do not have cats in their homes and have no direct contact with cats. Presumably those students with cats at home have cat dander on their clothes and transport it into the school environment.
The fact that one does not see a significant reduction in particles in the 3 to 10 micrometer range is surprising. To qualify as a MERV 8 filter, the filter must remove an average of 70% or more of the particles in the 3 to 10 micrometer range. Given this fact one would assume that the greatest particle reductions would be in these sizes. But this is not the case. Evidentially, the same aerodynamic and settling properties of the heavier particles make it so they do not reach the filters in sufficient numbers to affect the particle counts.
Another interesting point of the study is that even a MERV 8 filter can be relatively effective at reducing particles of PM2.5 and below. As mentioned above even though a MERV 8 filter does not have to meet a set level of efficiency on particles below 3 micrometers to be a MERV 8, they are effective at removing smaller particles. For example, it is typical that a MERV 8 filter will capture over 60% of the particles at 2 micrometers and 50% of the particles at 1 micrometer. Since much of the air inside a building is recirculated through the HVAC system whenever the unit is running, it is likely that most air will go through the filter multiple times and reduce particles on each pass.
This study is limited in that it only covers one school and only applies to the experience in one geographic area at one time of the year. However, it does give us an interesting glimpse at particles in schools and may explain some of the environmental reactions of students, faculty and administrative personnel in school enviroments.