Turn particle settling physics into practical site timing. Compare ventilation lift with humidity growth effects. Plan inspections, cleaning cycles, and controlled occupancy resets today.
Use upward airflow when supply or extraction pushes air upward. Choose a preset, then fine-tune values for your zone.
Sample scenarios to help verify entries and expected ranges.
| Scenario | Height (m) | Diameter (µm) | Density (kg/m³) | Temp (°C) | RH (%) | Upward air (m/s) | Expected behavior |
|---|---|---|---|---|---|---|---|
| Mold spore (typical) | 3.0 | 5.0 | 1100 | 20 | 50 | 0.00 | Settles within minutes to hours, depending on mixing. |
| Mold spore (small) | 3.0 | 2.5 | 1050 | 22 | 65 | 0.03 | May remain airborne longer under upward airflow. |
| Cement dust | 4.0 | 15.0 | 3150 | 25 | 40 | -0.02 | Faster deposition; downflow accelerates settling. |
The calculator estimates terminal settling velocity using a Stokes-flow approach with slip correction:
vs = ((ρp − ρa) · g · d² · Cc) / (18 · μ)
Air movement modifies deposition through an effective velocity: veff = vs − vair. Settlement time is then: t = (H / veff) · turbulence · safety. If veff ≤ 0, the model flags delayed settlement.
Airborne spores and fine dust can migrate far beyond the visible work area. Settlement time helps estimate when horizontal surfaces are likely to receive meaningful deposition after a disturbance such as demolition, sanding, or filter change-out. Longer settlement times generally increase cross-contamination risk, so isolation, cleaning frequency, and access control should be aligned with predicted timelines.
Larger or denser particles settle faster, while smaller particles remain suspended and are more sensitive to ventilation patterns. Even modest upward air velocity from supply diffusers, thermal plumes, or extraction imbalance can counter settling and delay deposition. Use the preset as a starting point, then adjust diameter and airflow to represent the dominant particle class and room behavior.
Relative humidity can change particle behavior through moisture uptake, aggregation, and surface interactions. The optional humidity growth setting provides a bounded adjustment that is useful for scenario testing. Treat the output as a planning estimate, and apply a safety factor when sensitive finishes, occupied zones, or regulatory containment requirements are present.
Settlement in real rooms is rarely a straight vertical drop. Mixing from people movement, equipment, and HVAC cycling re-suspends particles and increases deposition time. The turbulence factor represents this mixing penalty, while the safety factor adds conservatism for re-entry windows, inspection timing, and cleaning frequency. Higher factors are appropriate when measurements are limited or conditions vary through shifts.
Use a consistent “scenario set” to communicate controls and expected timelines. The examples below can be copied into daily reports and compared against site observations.
It estimates how long particles need to deposit from a release height under specified air properties, airflow, and mixing assumptions. Use it for planning cleanup timing and access controls, not as a compliance certificate.
If upward air velocity equals or exceeds particle settling velocity, the net downward motion becomes near zero. In that case, particles can remain suspended, travel with ventilation, and deposit unpredictably on distant surfaces.
Use the best available data: sampling reports, particle counter ranges, or typical values for your activity. When uncertain, test multiple diameters and report a conservative (longer) settlement time.
It represents indoor mixing, re-suspension, and non-ideal flow that slows net deposition. Increase it for active work, moving crews, fan cycling, or when air patterns are complex.
Enable it when you expect moisture uptake or stickier aerosols at higher humidity. Disable it for dry, mineral dust dominated conditions. It is a bounded scenario adjustment, not a species-specific laboratory model.
Set it above 1.0 when stakes are high: occupied areas, sensitive equipment, or uncertain inputs. Document why you increased it, and use the resulting time as the minimum window before inspection or re-entry.
CSV and PDF exports include your latest calculated inputs and outputs. They are generated on demand from stored session results, supporting quick attachment to daily logs, method statements, or closeout documentation.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.