LEL/UEL flammability assessment and ventilation requirements for confined space and indoor scenarios
How to use
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.