Why wildfire smoke demands a structured indoor response

Wildfire seasons now reach across continents, showering communities with fine particulate matter that slips through window cracks and ventilation systems. PM2.5 particles, small enough to penetrate deep into lungs and even enter the bloodstream, pose immediate risks for people with asthma, cardiovascular disease, or compromised immune systems. Yet even healthy adults experience headaches, irritated eyes, and sleeplessness when indoor concentrations climb. The instinct to buy the first air purifier on the shelf is understandable, but without a quantitative plan households often run devices in the wrong rooms, underestimate the volume of air they must clean, or expect miracles from filters that are far too small. A deliberate approach grounded in math turns scattered responses into a reliable indoor safe haven.

The planner accepts the most critical pieces of information: the size of the space you need to protect, the air exchange rate before and after sealing leaks, outdoor smoke levels, and the clean air delivery rate of each purifier. From those inputs, it calculates how quickly indoor air decays toward a safer concentration and whether your target threshold is even achievable given the equipment on hand. Families juggling evacuation decisions or caregivers protecting older adults can adjust the inputs to see how closing off certain rooms, adding another air cleaner, or lowering infiltration changes the timetable to breathable air. Instead of guessing whether the living room will be safe by bedtime, you receive an hour-by-hour forecast that demystifies the process.

Because PM2.5 behavior can be modeled as a first-order decay process, the planner uses the same exponential mathematics that engineers apply to cleanrooms and hospitals. That means the output reflects the compounding benefit of every additional clean air change per hour. Doubling your CADR does not merely double the cleaning rate; it shortens the time constant so aggressively that concentrations fall exponentially faster. Conversely, leaving a window cracked or a bathroom fan running can sabotage the effort by raising the effective air changes per hour with smoky outdoor air. By quantifying both effects, the planner encourages users to tackle infiltration and filtration together.

How the underlying decay model works

At the heart of the calculator is a mass-balance equation that treats your indoor space as a well-mixed volume. The change in indoor concentration over time is determined by the balance between smoky air entering the home and clean air generated by filters or recirculating HVAC systems. Mathematically, the rate of change obeys a simple differential equation with a closed-form solution. When the removal rate exceeds the infiltration rate, indoor concentrations decline exponentially toward an equilibrium that is lower than the outdoor environment. The speed of that decline depends on how many effective air changes per hour you can produce from portable purifiers and whether you can seal the building envelope to reduce smoky inflow.

The planner multiplies your home’s floor area by the ceiling height to determine volume, then converts clean air delivery rate (CADR) values from cubic feet per minute into equivalent air changes per hour. Every purifier you add increases the total CADR. If you have three devices rated at 300 cfm each, the tool converts them into 900 cfm, multiplies by 60 minutes, and divides by the volume to compute the filtration air change rate. That rate adds to the sealed infiltration rate to produce the total removal rate that appears in the results. Users can see how even modest purifiers add up quickly when combined or how a single large unit in a sealed bedroom can create a sanctuary during the worst smoke events.

The mathematics are transparent. The indoor concentration C(t) at time t hours after mitigation begins is given by the exponential decay:

$C(t)=C{\mathrm{eq}}_{}+(C{0}_{}-C{\mathrm{eq}}_{})e^{-\lambda t}$

Here, C₀ is the starting indoor concentration, C_eq is the equilibrium concentration achieved with the chosen combination of sealing and filtration, and λ is the total removal rate in air changes per hour. The planner computes λ by adding the sealed infiltration rate to the filtration rate. It sets C_eq equal to the outdoor concentration multiplied by the ratio of sealed infiltration to total removal, showing how the outdoor environment still influences indoor air even when purifiers run flat out. If you leave infiltration unchanged, the equilibrium becomes the outdoor concentration, and the equation simply predicts how fast indoor air catches up with the smoky air outside.

Worked example: turning a family room into a clean air shelter

Picture a two-story home where the family decides to shelter in a single 400-square-foot family room with an eight-foot ceiling during a severe smoke episode. Outdoor PM2.5 registers 300 µg/m³, while indoor air has already climbed to 220 µg/m³ after someone left a door open. The household owns two portable purifiers rated at 250 cfm each and a DIY box-fan filter rated at 500 cfm. They can tape plastic sheeting over the fireplace and weather-strip the patio door, reducing the infiltration rate from 1.0 air changes per hour to 0.3. Entering these numbers into the planner reveals that the combined CADR of 1,000 cfm translates into roughly 11 clean air changes per hour in the family room. The equilibrium concentration drops to about 7 µg/m³, and the time to fall below the WHO guideline of 15 µg/m³ is just over 25 minutes.

The hourly forecast table reinforces the strategy. Without mitigation, the room’s concentration would hover near 300 µg/m³ because infiltration keeps importing smoke. With sealing and filtration, the concentration plunges to 80 µg/m³ after 10 minutes, 20 µg/m³ after half an hour, and stabilizes near 7 µg/m³ within two hours. The plan also notes that the filter set will reach its rated life of 200 hours after roughly 8.3 days of continuous operation, reminding the family to keep spare filters on hand if they expect a prolonged smoke siege. Armed with those figures, the family can coordinate rest periods for vulnerable members and reassure neighbors that the room will become a refuge quickly.

Interpreting the hourly forecast table

The table beneath the calculator lists each hour up to the projection horizon you entered, comparing indoor concentrations with and without mitigation. The “no mitigation” column assumes you take no action beyond keeping windows closed, so indoor air simply drifts toward the outdoor level at a rate determined by the baseline air changes per hour. The second column captures the combined effect of sealing and filtration. If your mitigation plan cannot achieve the target threshold within the horizon, the result panel warns you so you can adjust the strategy. Many households discover that small purifiers barely dent concentrations in large, open-plan homes, prompting them to consolidate in a single room or supplement with additional units.

Exporting the CSV gives you a portable record of the forecast. Health clinics can archive the file to document how quickly waiting rooms clear after smoke intrusions. Emergency managers can integrate the data into incident action plans to justify distributing purifiers or setting up clean-air shelters. Even families preparing for upcoming smoke seasons can use the export as a checklist, ensuring they have the necessary devices and replacement filters before the first plume arrives.

Comparison of mitigation tactics

Not every mitigation tactic delivers the same bang for the buck. The table below summarizes common approaches, the benefits they provide, and the effort required. Use it alongside the calculator to decide which combination fits your home, budget, and physical capabilities.

Mitigation tactic comparison

Tactic	Primary benefit	When to prioritize
Seal leaks with painter’s tape and plastic	Reduces infiltration rate dramatically, lowering equilibrium concentration.	Older homes with fireplaces, through-the-wall air conditioners, or noticeable drafts.
Deploy HEPA or DIY purifiers sized to the room	Adds clean air changes per hour, speeding the decay toward safe levels.	Any space where vulnerable occupants will remain for several hours.
Run HVAC fan with upgraded MERV 13 filter	Circulates air through whole-house filtration without additional devices.	Homes with central systems capable of supporting higher-resistance filters.
Create a dedicated clean room	Concentrates resources, making it easier to reach strict health guidelines.	Situations with limited CADR, high outdoor smoke, or medically fragile individuals.

Using the result narrative

The result section interprets the numbers in plain language. It reports the total clean air changes per hour provided by your devices, the equilibrium concentration they can achieve, and the time required to hit your target. If the equilibrium is above your goal, the narrative recommends additional purifiers or more aggressive sealing. It also estimates how many days of continuous operation a filter can provide before reaching its rated life, helping you plan replacements or schedule rest periods. Because the planner assumes continuous operation once mitigation begins, you can rerun the scenario with shorter projection horizons if you plan to cycle devices on and off.

The narrative also highlights the risk of complacency. If the mitigation column shows a slow decline, you might be tempted to open windows prematurely, which could reintroduce smoke and reset the timeline. By quantifying the decay, the planner encourages households to wait until concentrations genuinely stabilize near healthy levels before resuming normal ventilation.

Limitations, assumptions, and safety considerations

The model assumes perfectly mixed air, which is a reasonable approximation for small rooms with fans running but less accurate for multilevel homes with stagnant corners. In real life, doorways, stairwells, and furniture create microenvironments where concentrations differ. Consider using additional sensors to verify air quality in critical rooms. The planner also assumes that portable purifiers maintain their CADR throughout the projection horizon. Filters clogged with ash or pet hair will decline in performance, so clean or replace them according to manufacturer guidance.

Another assumption is that the outdoor concentration remains constant over the horizon you choose. Wildfire plumes often fluctuate dramatically as winds shift. For extended events, rerun the planner with updated outdoor readings to keep forecasts accurate. Finally, the calculator does not account for internal sources such as cooking, candles, or vacuuming without HEPA filtration. Eliminate those activities during smoke episodes to prevent indoor spikes that undo your hard-won progress. The tool is designed to augment, not replace, guidance from local health departments. Pair it with evacuation alerts, real-time sensor data, and professional advice to keep your household safe.