Enter conditions to estimate safe reentry time
Calculator
Calculation log
Example data table
| Scenario | C0 (ppm) | Cs (ppm) | Half-life (min) | ACH | Mix | Buffer (min) | Estimated reentry (min) |
|---|---|---|---|---|---|---|---|
| Solvent wipe in small room | 300 | 30 | 70 | 4 | 0.75 | 15 | ≈ 164 |
| Coating cure with good exhaust | 220 | 25 | 55 | 10 | 0.90 | 10 | ≈ 87 |
| Confined space, minimal airflow | 180 | 20 | 80 | 1 | 0.60 | 30 | ≈ 257 |
Formula used
The calculator models concentration reduction as a first‑order process with two removal components: chemical decay and ventilation. The total removal rate is:
kdecay = ln(2) / halfLife
kvent = (ACH / 60) × mixingFactor
Time to fall from C0 to Cs is:
treentry = tbase + bufferMinutes
- Half-life and ACH are in minutes and air changes per hour.
- Mixing factor scales ventilation when airflow is uneven.
- Buffer adds conservative time for uncertainty.
How to use this calculator
- Enter the work area name to label your record.
- Set C0 (initial level) and Cs (required safe level) in ppm.
- Choose a half-life that matches your material or process.
- Add ventilation (ACH) and adjust mixing for dead zones.
- Include a buffer to align with site policy.
- Press Calculate to see reentry minutes and a timestamp.
- Keep “Add to log” checked to build an exportable log.
- Use CSV/PDF downloads to share results and archive them.
Airborne concentration changes on active sites
Reentry control is most critical after painting, solvent cleaning, adhesive curing, welding fume events, and confined-space maintenance. Air monitoring often shows sharp peaks at task completion, followed by gradual reduction as vapors disperse and fresh air replaces contaminated air. This calculator uses a first‑order approach that is commonly applied for short-duration clearance planning and shift coordination.
Key inputs that drive reentry minutes
The initial concentration C0 and the safe threshold Cs define the required reduction ratio (C0/Cs). A larger ratio increases time logarithmically, so reducing C0 through better capture, covering containers, or isolating the source can materially shorten downtime. Half-life represents chemical breakdown or settling; shorter half-life accelerates clearance even when ventilation is limited or intermittent.
Ventilation effectiveness and mixing quality
Air changes per hour (ACH) convert to a per-minute removal rate and are multiplied by a mixing factor. Mixing accounts for dead zones behind equipment, long corridors, and partially blocked exhaust paths. If you cannot confirm uniform airflow, lower the mixing factor and increase buffer minutes to keep planning conservative.
Operational buffers and permit documentation
A buffer helps cover uncertainties such as worker movement stirring residues, doors opening, temperature shifts, or variable exhaust performance. Many safety programs require documenting assumptions for reentry decisions. Exporting CSV/PDF logs supports toolbox talks, permit attachments, and repeatable planning across shifts. For multi‑trade schedules, saving each run lets you compare areas, verify assumptions, and justify staggered access for supervisors, spotters, or instrument checks before full crew return.
Interpreting results for scheduling
The output provides a single reentry time and an optional “safe at” timestamp when a work end time is provided. Use it to coordinate follow-on trades, inspection windows, and confined-space standby periods. When measured air data becomes available, compare readings to Cs and tune half-life or ventilation assumptions. If results are near a critical deadline, increase the buffer or require confirmatory measurements at the boundary time.
FAQs
1) What does “reentry time” mean here?
It is the estimated minutes for concentration to drop from C0 to Cs, including the optional buffer. It supports planning only and does not replace required monitoring, permits, or competent-person decisions.
2) How do I choose a half-life value?
Use product data, prior monitoring, or conservative estimates from similar tasks. If uncertain, pick a longer half-life and add buffer minutes. Update the value after you collect readings on comparable work.
3) What if I do not know the ACH?
Start with a low ACH for enclosed areas. If mechanical exhaust is verified and unobstructed, increase ACH. When airflow is intermittent, keep ACH conservative and rely on buffers until verified by measurements.
4) Why include a mixing factor?
Spaces rarely mix perfectly. The factor reduces ventilation effectiveness to reflect dead zones and short-circuiting. Lower it when obstacles, long runs, or partial barriers prevent even circulation of clean air.
5) Can I use this for dust or welding fumes?
It can help planning when levels decay roughly exponentially, but particulates may settle or resuspend. For higher-risk tasks, prioritize source control and verify clearance with air sampling or direct-read instruments.
6) Does the PDF count as compliance documentation?
The PDF is a calculation record that can support permits and briefings. Compliance depends on your site procedures, approvals, and any regulatory monitoring or clearance testing requirements for that work.