Select a substance or enter custom LEL/UEL values, then enter the measured or estimated atmospheric concentration. The tool calculates the percentage of LEL (%LEL) and determines the flammability zone. For confined space entry, monitor continuously — %LEL > 10% requires immediate action before entry.
Flammability assessment
Below LEL
Flammable range
Above UEL
Measured concentration—
% of LEL—
LEL / UEL—
Oxygen atmosphere assessment—
Select substance and enter concentration
Common substance reference — LEL / UEL at 20°C, 101.3 kPa
Substance
LEL (% v/v)
UEL (% v/v)
LEL (ppm)
Flash point
Methane
5.0
15.0
50,000
−187°C (gas)
Propane
2.1
9.5
21,000
−104°C (gas)
Butane
1.8
8.4
18,000
−60°C (gas)
Hydrogen
4.0
75.0
40,000
−253°C (gas)
Acetylene
2.5
100
25,000
−17°C (gas)
Ammonia
15.0
28.0
150,000
—
Carbon monoxide
12.5
74.0
125,000
—
Hydrogen sulfide
4.0
44.0
40,000
−60°C (gas)
Ethanol
3.3
19.0
33,000
13°C
Acetone
2.5
12.8
25,000
−20°C
Toluene
1.1
7.1
11,000
4°C
n-Hexane
1.1
7.5
11,000
−22°C
LEL/UEL values are at standard conditions (20°C, 101.3 kPa) unless noted. Temperature affects limits: roughly +8% LEL per 100°C increase. Oxygen-enriched atmospheres lower both limits significantly; oxygen-deficient atmospheres (<19.5%) may prevent ignition but create a different hazard. In confined spaces, atmospheric monitoring must use calibrated CGI equipment meeting ATEX/IECEx requirements. Action level: AS/NZS 2865 and OSHA 1910.146 specify entry not permitted when %LEL > 10%. Data sources: NFPA 68, SFPE Handbook, IEC 60079-20-1.
How to use
Estimate the ventilation rate required to dilute a gas or vapour release below the action level (10% LEL) or below the OEL. Enter the space volume, release rate, and target dilution factor to calculate the required air changes per hour (ACH) and flow rate.
Ventilation requirements
Required ventilation flow rate Q—
Required air changes per hour (ACH)—
Additional ACH required above existing—
Purge time to reach target %LEL—
Enter values above
Q = k × G / Ctarget, where G is release rate (m³/s), k is mixing factor, Ctarget is the target concentration (fraction of unity). ACH = Q × 3600 / V. Purge time t = −V/Q × ln(Cfinal/Cinitial) assuming well-mixed space. This tool provides a design estimate only — detailed dispersion modelling (CFD) is required for complex geometries, outdoor releases, or where stratification is possible. References: EN 60079-10-1:2021, NFPA 70; AS/NZS 60079.10.1.
How to Use
The two-zone near-field/far-field (NF/FF) model estimates steady-state contaminant concentrations in a room with a point source. The near-field zone (around the worker or source) will always have a higher concentration than the far-field (rest of room). Enter the source emission rate, room geometry, ventilation rate, and the inter-zone mixing rate, then press Calculate.
Inter-zone mixing rate (β) represents turbulent exchange between the near-field and far-field zones. A typical default of 0.1 m³/s is recommended for light indoor turbulence; increase to 0.3–0.5 m³/s for active processes or strong air movement.
Contaminant Source
Required to display concentrations in ppm
Room Geometry
m³
m³
Typically 1–4 m³ around the worker or source
Ventilation
Occupational Exposure Limit (Optional)
Results — Steady-State Concentrations
Near-Field CNF
—
mg/m³
Far-Field CFF
—
mg/m³
NF % of OEL
—
near-field vs limit
FF % of OEL
—
far-field vs limit
Time to 90% SS
—
minutes (far-field)
ACH (at Q)
—
air changes/hr
Interpretation
The near-field concentration is always higher than the far-field. Workers in the near-field zone are at greater risk than those elsewhere in the room. Increasing β (inter-zone mixing) or Q (ventilation rate) both reduce concentrations, but only increasing Q reduces the far-field concentration.
Concentration Build-up vs. Time
Two-zone NF/FF model (Nicas & Jayjock 2002). Steady-state: CFF = G/Q; CNF = G/β + G/Q. Build-up: CFF(t) = CFF,ss × (1 − e−Qt/VFF). Time to 90% steady-state: t90 = 2.3 × VFF/Q. All results assume steady-state emission and uniform mixing within each zone.
How to Use
The Gaussian plume model estimates ground-level centreline concentrations downwind of a stack or fugitive release under steady-state meteorological conditions. Select the atmospheric stability class based on wind speed and solar radiation (see the Pasquill-Gifford stability class guide below the inputs). Enter the effective stack height — this is the physical stack height plus any plume rise.
The model is appropriate for flat terrain, homogeneous meteorology, and distances from 50 m to approximately 10 km. It is not suitable for complex terrain, coastal sites, or low wind speed conditions (u < 1 m/s).
Source Parameters
m
Physical height + plume rise. Enter 0 for ground-level release.
Required to display concentrations in ppm
Meteorological Conditions
m/s
Receptor
m
Occupational / Environmental Limit (Optional)
Pasquill-Gifford Stability Class Guide
Surface wind (m/s)
Strong sun
Moderate sun
Overcast / night
Clear night
< 2
A
A–B
D
F
2–3
A–B
B
D
E–F
3–5
B–C
B–C
D
D–E
5–6
C
C–D
D
D
> 6
C
D
D
D
Adapted from Pasquill (1961) / Turner (1994). Class D applies for heavily overcast skies at any wind speed.
Results — Ground-Level Centreline Concentration
Concentration at x
—
mg/m³
% of Limit
—
concentration vs limit
Max Conc (centreline)
—
mg/m³
Distance to Max
—
m downwind
σy at x
—
m (horizontal spread)
σz at x
—
m (vertical spread)
Interpretation
Ground-level concentrations peak at a finite distance downwind then decrease. More stable conditions (E, F) produce a narrower, less dispersed plume with higher peak concentrations at greater distances. Tall stacks shift the peak further downwind and reduce near-source ground-level concentrations.
Ground-Level Centreline Concentration vs. Downwind Distance
Gaussian plume model: C(x) = Q / (π × σy × σz × u) × exp(−H² / (2σz²)). Dispersion coefficients from Pasquill-Gifford (Slade 1968 / Turner 1994): σy = a×x×(1+0.0001x)−0.5; σz varies by stability class. Model valid for flat terrain, steady meteorology, 50 m – 10 km from source. Not applicable for calm winds (< 1 m/s), complex terrain, or building downwash.