Fallow Time Calculator

The Fallow Time Calculator estimates required downtime between appointments based on ventilation rate, aerosol generation, room volume, and occupancy.

What Is a Fallow Time Calculator?

A fallow time calculator estimates how long a space or system should remain unused after an activity that creates risk or stress. In clinics and labs, it determines the minutes needed for indoor air to clear aerosols after a procedure. In agriculture, it helps plan time for soil to rest between crops.

The tool applies basic time and rate math. For indoor air, it uses the relationship between air changes and contaminant removal. For fields or process lines, it tracks set durations and buffers. The result is a clear, defensible waiting period you can communicate and schedule.

Fallow Time Formulas & Derivations

For indoor air, fallow time follows first-order decay in a well-mixed room. Ventilation and filtration remove contaminants at a rate set by air changes per hour. You choose a target removal percentage, and the equations produce the time needed.

• Core clearance time: t_minutes = −ln(1 − E) × 60 ÷ ACH, where E is target removal (for example, 0.99 or 0.999).
• Typical constants: For 99% removal, t ≈ 276 ÷ ACH minutes. For 99.9% removal, t ≈ 415 ÷ ACH minutes.
• Ventilation rate from volumetric flow: ACH = (Q_h ÷ V), where Q_h is hourly flow and V is room volume. Using flow in per-minute units: ACH = (CFM × 60) ÷ Volume_ft³.
• Adding filtration: Equivalent ACH from a cleaner is eACH = (CADR_h ÷ V). Total ACH = HVAC_ACH + eACH.
• Rounding rule: Always round up the result to the next whole minute. If using hours, round up to the next whole minute before formatting HH:MM.

The derivation assumes a single-zone, well-mixed space with constant removal rate. The natural log appears because concentration decays exponentially. If you prefer a fixed percentage table, many public health sources list minutes per ACH at common targets, which match these equations.

How the Fallow Time Method Works

The method converts air-cleaning capacity into a safe waiting period. It treats ventilation plus filtration as a steady removal process. You select a target removal percentage based on risk and guidance, then compute and apply the result.

• Identify the event that creates contamination risk, such as an aerosol-generating procedure.
• Calculate the total ACH from HVAC plus any portable air cleaners.
• Choose a target removal percentage, often 99% or 99.9% for higher-risk procedures.
• Compute fallow time using the formula and round up to whole minutes.
• Start the timer when the event stops and airflow conditions are stable, then restrict entry until the timer expires.

The same approach also supports simple duration planning, such as resting a field for a set number of days. In that case, you set a fixed duration and add buffers for weather or operations. The output remains a clear interval you can schedule.

Inputs, Assumptions & Parameters

Accurate inputs make the calculator useful in practice. Gather basic room data, airflow rates, and your risk target. When inputs are estimates, note the uncertainty and choose a conservative percentage or buffer.

• Room volume (ft³ or m³): Measure length × width × height, including ceiling height changes.
• Ventilation flow rate: Supply and/or exhaust airflow, in CFM or m³/h, converted to ACH.
• Filtration contribution: CADR from portable HEPA units or air cleaners, converted to eACH.
• Target removal percentage: Common choices are 95%, 99%, or 99.9%, based on risk.
• Output format: Minutes only or HH:MM; days or weeks for field-based durations.

Edge cases include very low ACH (long times), rooms with poor mixing, or doors opening frequently. For agriculture, consider weather delays and irrigation schedules. If conditions are variable, add a safety margin to your result.

Using the Fallow Time Calculator: A Walkthrough

Here’s a concise overview before we dive into the key points:

1. Measure or enter the room volume in your chosen units.
2. Enter HVAC airflow and convert to ACH, or enter ACH directly if known.
3. Add any air cleaner CADR to compute eACH and then total ACH.
4. Select a target removal percentage, such as 99.9% for high-risk procedures.
5. Compute t and round up to the next full minute.
6. Choose the output format, such as HH:MM, and save the result to your schedule.

These points provide quick orientation—use them alongside the full explanations in this page.

Real-World Examples

Dental treatment room: The room volume is 120 m³. HVAC provides 6 ACH. A portable HEPA unit adds CADR 400 m³/h. Total hourly clean air is 6 × 120 + 400 = 1,120 m³/h. Total ACH is 1,120 ÷ 120 = 9.33. For 99.9% removal, t ≈ 415 ÷ 9.33 = 44.5 minutes, so round up to 45 minutes (00:45 in HH:MM). What this means: After an aerosol-generating procedure, wait 45 minutes before the next patient enters.

Field fallow planning: You plan a rest period between vegetable crops. The target rest is 45 days to restore soil moisture. You expect 4 days of irrigation work that overlaps with rest and add a 7-day weather buffer. Net fallow time is 45 − 4 + 7 = 48 days. Format it as 6 weeks and 6 days for the farm calendar. What this means: Schedule the next planting 48 days after harvest to protect soil health.

Limits of the Fallow Time Approach

Fallow time estimates rely on simplified models and stable conditions. Real rooms may not mix perfectly, and real operations can interrupt airflow. Treat results as planning guidance and apply professional judgment.

• Well-mixed assumption may fail in large, irregular, or crowded rooms.
• Door openings, people movement, or equipment can change airflow mid-timer.
• ACH and CADR values may be measured with error or vary by speed setting.
• Pathogen viability, deposition, and resuspension are not directly modeled.
• For fields, weather variability and pest pressure can shift rest needs.

Mitigate these limits by verifying mixing with smoke tests, monitoring CO₂ as a proxy for air exchange, and adding safety margins. For critical settings, consult ventilation engineers or agronomists.

Units & Conversions

Units matter because time, flow, and volume must align. Keep consistent units when converting CFM to ACH, and present the result in a format your team uses. The table below summarizes common conversions and quick rules used in fallow time work.

Read the third column as the rule to apply. Keep units consistent, then format your final result as minutes or HH:MM for schedules, or days and weeks for longer durations.

Troubleshooting

If results look too short or too long, check your inputs and assumptions. Most issues trace back to unit mismatches, missing filtration, or unsteady airflow.

• Confirm units: CFM vs m³/h and ft³ vs m³ often cause errors.
• Recheck volume measurements, including ceiling height and alcoves.
• Verify device CADR at the actual fan setting used.
• Observe mixing with a simple smoke test; add fans if needed.
• If doors open often, add a buffer or reset the timer.

For agriculture, revisit weather assumptions weekly. If heavy rain or heat occurs, extend the rest period to protect yield and soil structure.

FAQ about Fallow Time Calculator

What removal percentage should I choose?

Use 99% for moderate risk and 99.9% for high-risk aerosol procedures or vulnerable occupants. Follow local guidance when available.

When should I start the fallow timer?

Start the timer immediately after the risk-generating activity stops and airflow is stable. If a door opens widely during the timer, consider restarting.

How do portable HEPA units change fallow time?

Convert each unit’s CADR to eACH and add it to HVAC ACH. Higher total ACH shortens the required time for the same removal percentage.

Can I format results as HH:MM for rotas?

Yes. Compute and round up to whole minutes, then display as HH:MM. For long intervals, use days or weeks to simplify planning.

Key Terms in Fallow Time

Air Changes per Hour (ACH)

The number of times the air in a room is replaced each hour. Higher ACH means faster dilution of contaminants.

Clean Air Delivery Rate (CADR)

A measure of how much clean air a purifier delivers per hour. Convert to eACH by dividing by room volume.

Equivalent ACH (eACH)

The effective air changes per hour contributed by filtration devices and other removal processes, added to HVAC ACH.

Removal Efficiency

The target fraction of contaminants to remove, such as 99% or 99.9%. Higher efficiency requires more time at the same ACH.

Well-Mixed Room

An assumption that air is uniformly mixed throughout the space. It enables exponential decay calculations for concentration.

Aerosol-Generating Procedure (AGP)

An activity that creates many fine particles, such as certain dental or medical procedures, increasing airborne risk.

Fallow Period

The time a room, line, or field stays unused to reduce risk or allow recovery, typically measured in minutes or days.

Time Constant

A measure related to the rate of decay in exponential processes. For air clearance, it depends on ACH and mixing.

References

Here’s a concise overview before we dive into the key points:

• CDC Environmental Infection Control: Appendix B. Air and Airborne Contaminant Removal
• ASHRAE Position Document on Infectious Aerosols
• WHO Roadmap to Improve and Ensure Good Indoor Ventilation
• CDC Ventilation in Buildings
• REHVA COVID-19 Ventilation and Building Services Guidance
• Scottish Dental Clinical Effectiveness Programme: Mitigation of AGPs in Dentistry

These points provide quick orientation—use them alongside the full explanations in this page.