Wildfire Smoke Infiltration Risk Calculator

Reviewed by: Dr. Mark Wickman

Understanding Wildfire Smoke Infiltration

Wildfires generate enormous quantities of fine particulate matter, and those microscopic particles can travel hundreds of miles from the flame front. When outdoor concentrations spike, people retreat indoors expecting safety, yet smoke invariably seeps through gaps, vents, or open doors. The ability of a building to keep smoke out depends on the rate at which outdoor air replaces indoor air, the efficiency of any filtration, and the natural processes that remove particles by settling on surfaces. The equation behind this calculator stems from a simple mass balance: the time derivative of indoor concentration equals the ingress from outside minus the combination of filtration and deposition removing particles from the indoor air. Solving the differential equation yields the steady‑state approach embodied in $$\mathrm{C\_\{in\}}=\mathrm{C\_\{out\}}\times \frac{\mathrm{AER}(1-\eta )}{\mathrm{AER}(1-\eta )+\mathrm{k\_\{dep\}}}(1-e^{-\left(\mathrm{AER}\right(1-\eta )+\mathrm{k\_\{dep\}})t})$$, where $\eta$ is filtration efficiency and $\mathrm{k\_\{dep\}}$ is deposition rate. The formula assumes constant conditions, but even this simplified representation captures how air exchange, filtration, and deposition interact to determine indoor pollution.

Understanding the parameters helps users gauge realistic outputs. The air exchange rate, expressed in air changes per hour (ACH), measures how often the volume of air in a building is replaced. Tight homes with closed windows may have an ACH as low as 0.2, whereas older or leaky structures may exceed 1. During smoky events people are often advised to close windows and doors, which reduces ACH but does not eliminate infiltration entirely. Filtration efficiency represents the fraction of particles removed by a device such as a HEPA filter. A clean, high‑quality filter in a properly sized purifier might achieve 0.99 efficiency, whereas a furnace filter designed for dust may only remove 0.3 of incoming particles. The deposition rate accounts for gravitational settling and surface impaction; in most residences this ranges from 0.1 to 1 per hour depending on particle size and household activity. By combining these variables, the calculator estimates the indoor concentration after a specified duration.

The second part of the calculation assesses risk using a logistic function. Public health agencies often classify PM2.5 levels above 35 µg/m³ as unhealthy for sensitive groups. To translate the modelled indoor concentration into a probability of unhealthy air, the calculator applies $$P=\frac{1}{1+e^{\frac{-(C\_\{in\}-35)}{10}}}$$. Values near zero mean the home is likely safe, while numbers approaching one signal high risk. Although the logistic parameters here (35 and 10) are chosen for clarity rather than epidemiological precision, they provide an intuitive risk scale: at 35 µg/m³ the curve crosses 50%, and each additional 10 µg/m³ roughly doubles the odds of unhealthy conditions.

To illustrate, consider an area blanketed by thick smoke with outdoor concentration 200 µg/m³. A resident closes windows, achieving an air exchange rate of 0.3, runs a portable purifier with 0.9 efficiency, and experiences a deposition rate of 0.2 h⁻¹ from settled dust. After two hours the calculator predicts indoor concentration of roughly 40 µg/m³, translating to a 58% chance of unhealthy air. If the event lasts eight hours without additional filtration, the indoor level climbs toward 60 µg/m³, with risk exceeding 80%. Such insights can prompt strategic actions: adding a second purifier, sealing gaps, or relocating to a cleaner space.

For decision makers, the tool can evaluate building upgrades. Suppose a school experiences frequent smoke intrusions. Administrators might compare scenarios with and without high‑efficiency filters. Entering an outdoor concentration of 150 µg/m³, an ACH of 1.0 for the existing HVAC system, a current filter efficiency of 0.4, a deposition rate of 0.3, and a three‑hour duration yields an indoor level of roughly 70 µg/m³ and a 90% risk. Upgrading to filters with 0.9 efficiency drops the level to 25 µg/m³, cutting risk to 25%. This dramatic improvement can inform investment decisions and communicate the benefits to parents and staff.

Beyond immediate health concerns, understanding infiltration informs maintenance schedules. The same equation reveals how long a single purifier can maintain safe levels before a filter change is necessary. High outdoor concentrations increase the mass of captured particles, shortening filter lifespan. Coupling this calculator with a filter cost analysis can optimize inventory and budgeting for smoke seasons. It also aids in emergency planning: knowing that infiltration will exceed safe thresholds after three hours might lead a community center to stock additional filters or create rotation schedules for clean-air shelters.

Below is a table translating air exchange rate to qualitative leakage categories. Structures with very low ACH values tend to be energy efficient and retain indoor air better, but they may accumulate indoor pollutants; conversely, very high ACH values dilute indoor emissions yet allow more smoke to enter during wildfire events.

While this calculator is intentionally simple, it underscores the dynamics that determine indoor exposure during smoke episodes. Researchers have developed advanced models incorporating time-varying conditions, chemical transformations, and size-resolved particle physics, but those require specialized expertise and data. Here, everything runs offline in your browser, making it accessible during power outages or evacuations when internet access may be limited. Users are encouraged to experiment with hypothetical values: How much does taping window seams reduce infiltration? What happens if you run the purifier continuously for 24 hours? How quickly does air clean once the outdoor concentration drops? By exploring scenarios, occupants gain a tangible sense of control, empowering them to protect their health.

As smoke seasons lengthen due to climate change, understanding indoor air dynamics becomes increasingly important. The mere presence of a purifier does not guarantee safety; placement, maintenance, and capacity matter. Moreover, vulnerable populations like children, seniors, and those with respiratory conditions may require stricter thresholds. Pair this tool with carbon monoxide detectors, emergency kits, and evacuation plans for comprehensive preparedness. Though the calculator cannot predict every nuance of indoor air behavior, its transparent equations and adjustable inputs offer a solid foundation for informed decision making, demonstrating how science, math, and practical action intertwine to keep homes safe in a smoky world.

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