Why indoor air exchanges deserve deliberate planning

Respiratory pandemics, wildfire smoke, and urban pollution have thrust ventilation into the mainstream. Yet homeowners, school administrators, and community leaders often have limited tools to quantify the effectiveness of their existing systems. Common rules of thumb—like “open a window” or “run the fan on high”—provide little clarity on whether the resulting airflow meets recommended air changes per hour (ACH) for infection control or smoke dilution. The Indoor Air Exchange Upgrade Planner turns the physics of volumetric flow into actionable numbers so decision makers can stage interventions, justify equipment purchases, and communicate the benefits of layered strategies.

Unlike heating or cooling, ventilation is invisible. People experience stale air as fatigue, headaches, or that intangible “stuffy room” feeling, but they rarely know how many cubic feet per minute (CFM) their space actually exchanges. CO₂ monitors help but do not quantify clean air delivery from portable filters or infiltration. Public health guidance from agencies such as the CDC and ASHRAE emphasizes target ACH levels ranging from 4–6 for classrooms to 12 or more for health-care isolation rooms. Without a planner, facility managers must guess whether a combination of mechanical ventilation, HEPA purifiers, and outdoor air provides enough protection.

This tool fills the gap by combining fundamental equations with a practical interface. Users specify room dimensions, mechanical supply rates, background infiltration, and filtration CADR. They can also model events such as choir practice or staff meetings by entering occupancy, infectious individuals, and exposure duration. The planner outputs current ACH, the additional CFM required to reach the target, and the expected infection probability using the Wells–Riley equation. It also builds comparison scenarios—for example, adding a window fan or an extra purifier—so planners can prioritize interventions that deliver the biggest risk reduction per dollar.

From room volume to infection probability

The calculations start with geometry. A rectangular room with length L, width W, and height H has a volume V equal to $V=L\times W\times H$ cubic feet. Every source of clean air—mechanical ventilation, infiltration, and filtration—contributes a flow rate expressed in cubic feet per minute. The total clean air delivery rate is therefore $Q=Q_{\text{mech}}+Q_{\text{inf}}+Q_{\text{fil}}$. To convert to ACH, the tool multiplies by 60 minutes and divides by room volume: $\mathrm{ACH}=\frac{60}{V}\times Q$. Meeting a target ACH therefore requires achieving a total flow $Q_{\text{target}}=\frac{\mathrm{ACH}}{60/V}$, or more simply, $Q_{\text{target}}=\mathrm{ACH}\times V/60$.

To translate airflow into health risk, the planner uses the Wells–Riley model, a classic epidemiological framework. If an infectious person emits q quanta per hour, susceptible occupants breathe at rate p cubic meters per hour, and the exposure lasts t hours, the probability that any one susceptible person becomes infected is $P=1-e^{-\frac{I_{\text{infectious}}}{\times }}$, where the 1.699 factor converts cubic feet per minute into cubic meters per hour. The exponential decay captures how added clean air dilutes infectious aerosols. The planner reports infection probability as a percentage so users can compare scenarios at a glance.

Because most buildings host more than one person, the tool also highlights clean air per capita. Dividing the total CFM by occupancy yields ventilation per person, a metric that indoor air quality researchers use to gauge comfort and cognitive impacts. Studies suggest that cognitive performance improves once per-person ventilation exceeds roughly 20–25 CFM, with diminishing returns beyond 40–50 CFM. The planner flags how each intervention shifts this metric, enabling organizations to balance infection risk reduction with productivity goals.

Worked example: upgrading a multipurpose room

Imagine a 20-by-15-foot community room with 9-foot ceilings used for after-school tutoring and neighborhood meetings. The existing HVAC system supplies 180 CFM of outdoor air. Infiltration adds an estimated 40 CFM through door cracks and wall leakage. The center owns one HEPA purifier rated at 250 CADR. Ten people typically use the room for two-hour sessions, and during respiratory virus season the organizers want to plan for the possibility of one infectious person. Participants speak in normal voices, so a quanta emission rate of 25 per hour is reasonable, and an average breathing rate of 0.6 m³/hour matches light activity. The goal is to reach 8 ACH, consistent with upper-end school guidance.

The planner calculates the room volume at 2,700 cubic feet. Current clean air delivery totals 470 CFM (180 + 40 + 250). Converting to ACH yields (470 × 60 ÷ 2,700) ≈ 10.4 ACH, which already exceeds the target. Clean air per person sits at 47 CFM, supporting both infection control and cognitive performance. The Wells–Riley computation estimates that each susceptible person faces a 5.1 percent infection probability during the two-hour event. The result pane highlights that no additional CFM is required to hit the target, yet scenario comparisons reveal further opportunities to cut risk.

For instance, adding a second HEPA purifier with 250 CADR lifts clean airflow to 720 CFM and drops infection probability to 3.4 percent. Opening windows with a box fan that adds 300 CFM of outdoor air reduces risk even further. Combining both measures slashes the probability to 2.2 percent. The table of scenarios quantifies these improvements, helping organizers decide whether purchasing another purifier or deploying fans is worth the noise and logistical effort.

Comparison of upgrade strategies

To support decision-making, the planner assembles a comparison table with four strategies: baseline, add HEPA, add window fan, and add both. Each row shows the clean air flow in CFM, the equivalent ACH, the per-person airflow, and the predicted infection probability. A final column indicates whether the strategy meets the target ACH. Even when the baseline already meets the target, alternate rows illustrate how layered controls compound benefits. Users can download the table to share with stakeholders or to support grant applications for ventilation improvements.

The comparative view demonstrates that while the base configuration satisfies ACH guidelines, additional filtration and ventilation can still provide measurable benefits. This matters when considering vulnerable populations, such as immunocompromised students or elders. The table also helps manage expectations: achieving 2 percent infection probability requires significantly more airflow than reaching 5 percent. Facility managers can weigh the marginal gains against budget, noise tolerance, and ease of deployment.

Limitations and assumptions

Several simplifying assumptions underpin the planner. It treats the room as well-mixed, meaning clean air instantly dilutes contaminants evenly. Real spaces often have dead zones or short-circuiting where supply air leaves before mixing. Users can approximate this by modestly increasing the target ACH. The Wells–Riley model does not account for masks, vaccination, or aerosol settling; adding masks effectively reduces quanta emission and inhalation rates, which users can simulate by lowering the quanta or breathing inputs. Filtration CADR values assume properly maintained filters and unobstructed airflow. Dust buildup or clogged prefilters can reduce performance. Similarly, infiltration estimates are rough; wind, stack effect, and door openings make natural ventilation highly variable.

The calculator also focuses on respiratory pathogens, but the same airflow improvements help with wildfire smoke and general comfort. For smoke events, users can set the infectious count to zero and focus on CADR and ACH values. Because particle infiltration is undesirable during smoke, the “add window fan” strategy may not apply; the downloadable table can be edited to swap in alternative strategies such as sealing leaks or adding recirculating filtration. Finally, the tool does not estimate humidity or temperature impacts, which may be important for occupant comfort. Despite these caveats, the planner provides a transparent framework for reasoning about indoor air exchanges. It brings the language of ACH, CFM per person, and infection probability into everyday decision-making, empowering schools, workplaces, and households to act with confidence.