Toxic & Flammable Gas Reference Data – OSHA, NIOSH
Engineering reference for industrial gas detection and safety system design, with TWA, STEL, LEL limits, relative density.
Why This Reference Matters
This page provides indicative engineering reference data commonly used during:
- Gas detector selection, measuring range, threshold design and layout
- Preliminary hazard analysis (HAZID / HAZOP)
- Compliance review and system sizing
- Concept and FEED stage documentation
Values shown are typical industry references compiled from OSHA, NIOSH, ACGIH and manufacturer technical literature. Final project decisions must follow site-specific risk assessment, design criteria, application requirement and applicable standards.
Installation Height Logic (important):
- Lighter than air (<1.0): Install detectors near ceiling
- Heavier than air (>1.0): Install detectors near floor
- Near air density (~1.0): Install at breathing zone (1.2–1.5 m)
| Gas | LEL (% vol) | UEL (% vol) | Density (Air=1) | Detector Location | Action |
|---|---|---|---|---|---|
| Methane (CH₄) | 5 | 15 | 0.55 | Near ceiling | Calculate |
| Hydrogen (H₂) | 4 | 75 | 0.07 | Near ceiling | Calculate |
| LPG (Propane / Butane) | 2.1 | 9.5 | 1.8 | Near floor | Calculate |
| CNG / PNG | 5 | 15 | 0.55 | Near ceiling | Calculate |
| Hydrogen Sulfide (H₂S)
(Also toxic) |
4.3 | 45.5 | 1.19 | Near floor | Calculate |
| Ammonia (NH₃)
(Also toxic) |
15 | 28 | 0.6 | Near ceiling | Calculate |
| Formaldehyde
(Also toxic) |
7 | 73 | 1.07 | Breathing zone | Calculate |
| Hydrogen Cyanide (HCN)
(Also toxic) |
5.6 | 40 | 0.94 | Breathing zone | Calculate |
| Phosphine
(Also toxic) |
1.8 | 98 | 1.2 | Breathing zone | Calculate |
| Acetone
(Also toxic) |
2.6 | 12.8 | 2 | Near floor | Calculate |
| IPA (Isopropyl Alcohol)
(Also toxic) |
2 | 12 | 2.1 | Near floor | Calculate |
| Toluene
(Also toxic) |
1.2 | 7.1 | 3.1 | Near floor | Calculate |
| Hexane
(Also toxic) |
1.2 | 7.5 | 3 | Near floor | Calculate |
| Petrol Vapour | 1.4 | 7.6 | 3 | Near floor | Calculate |
| Diesel Vapour | 0.6 | 7.5 | 4 | Near floor | Calculate |
| VOC / Solvent Vapour (Generic)
(Also toxic) |
1–3 | 6–12 | >1 | Near floor | Calculate |
| Styrene
(Also toxic) |
1.1 | 6.1 | 3.6 | Near floor | Calculate |
| Gas | TWA | STEL | IDLH | Density | Detector Location | Action |
|---|---|---|---|---|---|---|
| Carbon Monoxide (CO) | 25 | 200 | 1200 | 0.97 | Breathing zone | Calculate |
| Carbon Dioxide (CO₂) | 5000 | 30000 | 40000 | 1.53 | Breathing zone | Calculate |
| Oxygen (O₂) | 19.5–23.5 % | — | — | 1.11 | Breathing zone | Calculate |
| Nitrogen (N₂) | — | — | — | 0.97 | Ceiling (deficiency) | Calculate |
| Nitrous Oxide (N₂O) | 25 | — | — | 1.53 | Breathing zone | Calculate |
| Hydrogen Sulfide (H₂S)
(Also flammable) |
10 | 15 | 100 | 1.19 | Near floor | Calculate |
| Ammonia (NH₃)
(Also flammable) |
25 | 35 | 300 | 0.6 | Near ceiling | Calculate |
| Formaldehyde
(Also flammable) |
0.75 | 2 | 20 | 1.07 | Breathing zone | Calculate |
| Hydrogen Cyanide (HCN)
(Also flammable) |
10 | — | 50 | 0.94 | Breathing zone | Calculate |
| Phosphine
(Also flammable) |
0.3 | 1 | 50 | 1.2 | Breathing zone | Calculate |
| Acetone
(Also flammable) |
— | — | — | 2 | Near floor | Calculate |
| IPA (Isopropyl Alcohol)
(Also flammable) |
— | — | — | 2.1 | Near floor | Calculate |
| Toluene
(Also flammable) |
— | — | — | 3.1 | Near floor | Calculate |
| Hexane
(Also flammable) |
— | — | — | 3 | Near floor | Calculate |
| VOC / Solvent Vapour (Generic)
(Also flammable) |
— | — | — | >1 | Near floor | Calculate |
| Chlorine (Cl₂) | 0.5 | 1 | 10 | 2.5 | Near floor | Calculate |
| Ozone (O₃) | 0.1 | 0.3 | 5 | 1.6 | Breathing zone | Calculate |
| Sulfur Dioxide (SO₂) | 2 | 5 | 100 | 2.26 | Near floor | Calculate |
| Nitrogen Dioxide (NO₂) | 3 | 5 | 20 | 1.6 | Breathing zone | Calculate |
| Nitric Oxide (NO) | 25 | — | 100 | 1.04 | Breathing zone | Calculate |
| Phosgene | 0.1 | — | 2 | 3.4 | Near floor | Calculate |
| Hydrofluoric Acid (HF) | 3 | 6 | 30 | 0.92 | Breathing zone | Calculate |
| Hydrochloric Acid (HCl) | 5 | — | 50 | 1.3 | Breathing zone | Calculate |
| Styrene
(Also flammable) |
— | — | — | 3.6 | Near floor | Calculate |
ppm: —
% Volume: —
% LEL: —
Online Gas Concentration Conversion Calculator
PPM ↔ % Volume Converter
ppm: —
% Volume: —
1% volume = 10,000 ppm
Gas Concentration ↔ %LEL Converter
ppm: —
% Volume: —
% LEL: —
%LEL is referenced to gas-specific Lower Explosive Limit (% volume).
Applicable only for flammable gases. LEL values are gas-specific.
Common Engineering Mistakes
- Using ppm alarms for flammable gases instead of %LEL
- Installing detectors at wrong height due to ignoring gas density
- Mixing OSHA TWA with ACGIH STEL incorrectly
- Using laboratory limits for outdoor or ventilated areas
- Ignoring response time and cross-sensitivity during selection
- Improper low voltage earthing, leading to static charges accumulation
- Improper cable selection and installation
APPLICATION SECTORS
This reference is typically used in:
- Oil & Gas production & processing
- Refineries & petrochemical plants
- Chemical manufacturing units
- Fertilizer & ammonia plants
- Warehouses & cold storage
- Battery energy storage systems (BESS)
- Wastewater & utility plants
- LNG / LPG facilities
STANDARDS & GUIDELINES (REFERENCE)
- OSHA Occupational Exposure Limits
- NIOSH IDLH Guidelines
- ACGIH TLV recommendations
- IEC / EN gas detection standards
- NFPA flammable gas practices
- ATEX / IECEx installation principles
Technical Disclaimer
Data provided is for engineering reference only. Final gas detection design, alarm limits, installation height and compliance shall be confirmed through project-specific risk assessment, applicable codes and authority requirements.