Clean indoor air, whatever the outdoor reading.

The problem

Beijing regularly sees outdoor fine-particle pollution (PM2.5) at hazardous levels, sometimes above 300 µg/m³ and on occasion beyond 500. Those particles come largely from coal combustion and traffic, and they are small enough to reach deep into the lungs.

In the winter of 2013, students at the International School of Beijing (ISB) measured indoor air quality as part of a science class and found PM2.5 inside classrooms, hallways and large spaces exceeding 150 µg/m³ on the worst outdoor days. For a school responsible for the health of students, faculty and staff, that was unacceptable.

What was going wrong

A review of the HVAC system pointed to two linked causes:

• Inadequate filtration. The existing fresh-air handlers used two filter stages (F6 and F8), which could not bring heavily polluted outdoor air down to acceptable indoor levels.
• Negative building pressure. On bad days the fresh-air units were shut off to avoid pulling in dirty air. That left the building under negative pressure, so polluted air rushed in through doors and gaps around operable windows instead.

The solution

During June and July 2013, the school replaced all 35 air handlers serving fresh-air intakes across the 51,000 m² campus:

• Three-stage filtration. New modular central-station units added a HEPA H14 final stage (removing more than 99.995% of particles down to 0.3 µm) behind F6 and F8 filters.
• Positive pressurization. Roughly 30% more airflow to classrooms and open spaces put the whole building under positive pressure.
• Targeted over-pressurization. Eight high-traffic double-door entrances and every stairwell were pressurized further, limiting infiltration at doors and keeping first-floor air from migrating upward through the three-story building.
• A tighter envelope. All operable windows were removed.
• Efficient control. Every new unit runs on a variable-frequency drive. Moving more air through more filtration increased energy use by only about 10%.

The upgrade cost approximately RMB 10.7 million (about US$1.77 million). Replacement of the remaining 24 return-air handlers was planned for the following summer.

How results were measured

Facilities staff took readings with calibrated aerosol monitors at 26 indoor locations and one outdoor location, three times a day, timed just after the morning arrival, lunch and dismissal. They logged 17 days before the upgrade (February to March 2013) and 24 days after (July to August 2013). Outdoor readings were corrected for relative humidity, which inflates summertime measurements.

High-school students then ran an independent study over six days in late August 2013, sampling 32 indoor and two outdoor locations with monitors they field-calibrated against the U.S. Embassy's reference station.

The results

With average outdoor PM2.5 essentially unchanged between the two periods, average indoor levels fell from 16.7 to 3.2 µg/m³. Indoor air went from carrying about 21% of the outdoor particle load to about 4%.

Just as important, indoor levels stopped tracking the weather. After the upgrade, the indoor average stayed below 9 µg/m³ even as outdoor readings swung between 5 and 274 µg/m³, and spaces targeted directly by the upgrade improved the most. The student measurements told the same story.

The takeaway

Buildings in heavily polluted cities can protect occupants without depending on the outdoor forecast. The combination that worked here (high-efficiency filtration at every fresh-air intake, enough airflow to hold the building at positive pressure, and a tighter envelope) is exactly the kind of system-level fix that only holds up when airflow and pressure relationships are designed, balanced and verified together.