Which gases are toxic or flammable at work? IDLH, PEL, REL and LEL for 17 gases met in confined spaces, sewers, plants, fires and oil and gas, and the sensor that reads each
The gases that kill at work fall into two lists that overlap. The toxic list is headed by hydrogen sulfide (NIOSH IDLH 100 ppm, OSHA ceiling 20 ppm), carbon monoxide (IDLH 1,200 ppm, PEL 50 ppm), chlorine (IDLH 10 ppm, ceiling 1 ppm), nitrogen dioxide (IDLH 13 ppm), hydrogen cyanide (IDLH 50 ppm), phosphine (IDLH 50 ppm), sulfur dioxide (IDLH 100 ppm), ammonia (IDLH 300 ppm) and benzene (IDLH 500 ppm, a carcinogen with a 1 ppm PEL). The flammable list is headed by the gases with the widest ranges and the lowest limits: hydrogen (4.0 to 75.6 percent by volume), acetylene (2.5 to 100), methane (5.0 to 15.0), propane (2.1 to 9.5), pentane (1.5 to 7.8) and hexane (1.1 to 7.5), with hydrogen sulfide, carbon monoxide, ammonia, hydrogen cyanide and the solvent vapors on both lists. Carbon dioxide, nitrogen and methane also kill by displacing oxygen, with no toxic limit low enough to warn. OSHA’s confined-space rule turns all of this into three tests, in order: oxygen 19.5 to 23.5 percent, flammables under 10 percent of the LEL, and every toxic under its limit with nothing at an IDLH level (1910.146(b) and (d)(5)). The table on this page gives the numbers for 17 gases with the sensor that reads each, and the band lists the monitors by gas.
RKI Instruments 72-PAX-D GX-3R Pro 4-Gas Monitor, LEL/O2/H2S/CO, Li-ionFour-gas monitor: the oxygen, LEL, H2S and CO channels that cover most of the tableWhy one table and not two. A buyer asks which gases are toxic and which are flammable, and the answer for most gases met in confined spaces is both, at concentrations a thousand times apart. Hydrogen sulfide is flammable at 4 percent by volume (40,000 ppm) and immediately dangerous to life at 100 ppm; carbon monoxide burns at 12.5 percent and is IDLH at 1,200 ppm. So the toxic sensor, reading in ppm, is always the first warning for a gas on both lists, and the LEL sensor matters for the gases with no toxic limit low enough to help: methane, propane, hydrogen, which asphyxiate or burn but do not poison. The third group, the asphyxiants, is read by the oxygen sensor. That is why the four-gas monitor carries one of each kind of sensor and why OSHA’s testing order is oxygen, then flammables, then toxics.
17 gases met at work: the OSHA PEL, NIOSH REL and IDLH, flammable range, where each is found and the sensor that reads it
| Gas | Formula | OSHA PEL | NIOSH REL | NIOSH IDLH | LEL / UEL, percent by volume | Where it is met | Sensor |
|---|---|---|---|---|---|---|---|
| Hydrogen sulfide | H2S | C 20 ppm, 50 ppm 10-minute peak | C 10 ppm (10 min) | 100 ppm | 4.0 / 44.0 | Sewers, wet wells, digesters, manure pits, sour crude and gas | Electrochemical |
| Carbon monoxide | CO | TWA 50 ppm | TWA 35 ppm, C 200 ppm | 1,200 ppm | 12.5 / 74 | Engines indoors, furnaces, welding, fires, confined spaces near engines | Electrochemical |
| Carbon dioxide | CO2 | TWA 5,000 ppm | TWA 5,000 ppm, ST 30,000 ppm | 40,000 ppm | Nonflammable | Fermentation, dry ice, beverage systems, digesters, landfills, fire suppression | Infrared |
| Ammonia | NH3 | TWA 50 ppm | TWA 25 ppm, ST 35 ppm | 300 ppm | 15 / 28 | Refrigeration machine rooms, fertilizer, wastewater, cleaning | Electrochemical |
| Chlorine | Cl2 | C 1 ppm | C 0.5 ppm (15 min) | 10 ppm | Nonflammable, oxidizer | Water and wastewater disinfection, bleach plants, pools | Electrochemical |
| Sulfur dioxide | SO2 | TWA 5 ppm | TWA 2 ppm, ST 5 ppm | 100 ppm | Nonflammable | Sour gas plants, smelters, combustion, pulp mills | Electrochemical |
| Nitrogen dioxide | NO2 | C 5 ppm | ST 1 ppm | 13 ppm | Noncombustible, accelerates burning | Diesel exhaust, welding, silos (silo gas), nitric acid | Electrochemical |
| Hydrogen cyanide | HCN | TWA 10 ppm (skin) | ST 4.7 ppm (skin) | 50 ppm | 5.6 / 40.0 | Fire overhaul (burning plastics), plating, fumigation | Electrochemical |
| Phosphine | PH3 | TWA 0.3 ppm | TWA 0.3 ppm, ST 1 ppm | 50 ppm | 1.79 / — | Grain fumigation, semiconductor, metal phosphide reactions | Electrochemical |
| Benzene | C6H6 | 1 ppm TWA, 5 ppm STEL (1910.1028) | Ca, TWA 0.1 ppm, ST 1 ppm | Ca 500 ppm | 1.2 / 7.8 | Refineries, petrochemical, coke ovens, gasoline | PID (10.0 eV), tube |
| Toluene | C7H8 | TWA 200 ppm, C 300 ppm, 500 ppm peak | TWA 100 ppm, ST 150 ppm | 500 ppm | 1.2 / 7.0 (RKI chart) | Paints, coatings, adhesives, printing, fuels | PID |
| Xylene | C8H10 | TWA 100 ppm | TWA 100 ppm, ST 150 ppm | 900 ppm | — (RKI chart not listed) | Paints, coatings, printing, cleaning | PID |
| Hexane | C6H14 | TWA 500 ppm | TWA 50 ppm | 1,100 ppm (10 percent LEL) | 1.1 / 7.5 (RKI chart) | Solvents, adhesives, extraction, fuels | PID, catalytic LEL |
| Methanol | CH3OH | TWA 200 ppm | TWA 200 ppm, ST 250 ppm (skin) | 6,000 ppm | 6.0 / 36.0 (RKI chart) | Fuels, antifreeze, solvents, biodiesel | PID (11.7 eV) |
| Methane | CH4 | Simple asphyxiant, no PEL | No REL | Not listed | 5.0 / 15.0 (RKI chart) | Natural gas, sewers, landfills, digesters, mines | Catalytic LEL, infrared |
| Propane | C3H8 | TWA 1,000 ppm | TWA 1,000 ppm | 2,100 ppm (10 percent LEL) | 2.1 / 9.5 | Fuel, LPG, forklifts, heaters | Catalytic LEL |
| Hydrogen | H2 | Simple asphyxiant, no PEL | No REL | Not listed | 4.0 / 75.6 (RKI chart) | Battery charging, refineries, electrolysis | Catalytic LEL (not infrared) |
Where the NIOSH Pocket Guide and RKI’s chart differ (hydrogen 75 against 75.6 percent UEL), the source is marked; RKI notes the limits are empirically determined and authorities quote slightly different figures.
Source: NIOSH Pocket Guide to Chemical Hazards (current online edition) (PEL, REL, IDLH and the LEL/UEL where marked NIOSH or unmarked); RKI Instruments FAQ page (rkiinstruments.com/resources/faqs, rkiinstruments.com) LEL and UEL chart where marked RKI chart; 29 CFR 1910.1000 Tables Z-1 and Z-2; 29 CFR 1910.1028 for benzene. C is a ceiling, ST a 15-minute short-term limit, TWA an 8-hour average, Ca a NIOSH potential occupational carcinogen. Methane and hydrogen have no exposure limit; they asphyxiate by displacing oxygen and burn.
The three ways a gas kills, and the sensor for each
Which gases turn up where, by the sources named
- Confined spaces (sewers, wet wells, digesters, manure pits): hydrogen sulfide, methane, carbon dioxide and oxygen deficiency; RKI Instruments’ landfill page gives raw landfill gas as 45 to 60 percent methane, 40 to 60 percent carbon dioxide, 0 to 1.0 percent sulfides and 0.1 to 1.0 percent oxygen.
- Engines indoors and fires: carbon monoxide and nitrogen dioxide from diesel exhaust and welding; hydrogen cyanide off-gassing from burned plastics, which RKI names for its firefighter configurations (HCN 0 to 30.0 ppm, NO2 0 to 20.0 ppm).
- Oil and gas and refineries: hydrogen sulfide from sour crude and gas, hydrocarbon vapor above the LEL, benzene, sulfur dioxide at sour gas plants, and hydrogen that fools a standard CO sensor (RKI’s hydrogen-compensated CO sensor).
- Water and wastewater plants: chlorine and sulfur dioxide at disinfection, ammonia, hydrogen sulfide and methane at the headworks and digesters, oxygen deficiency in vaults.
- Cold storage and food: ammonia in refrigeration machine rooms (PEL 50 ppm, IDLH 300 ppm), carbon dioxide from fermentation and dry ice.
- Grain and agriculture: phosphine from fumigation (PEL 0.3 ppm, IDLH 50 ppm), nitrogen dioxide (silo gas), hydrogen sulfide and methane from manure pits.
- Paint, coatings, printing and cleaning: toluene, xylene, hexane, methanol, acetone and other solvent vapors, read by PID with the correction factor for each.
Set points beside the limits
The alarm points the two lines ship are set below the OSHA and NIOSH limits in the table above; here they are as printed.
Factory alarm set points as Honeywell BW and MSA print them (low, high, TWA, STEL)
| Model (source) | O2 | LEL | CO | H2S | Other |
|---|---|---|---|---|---|
| Honeywell BW Clip (datasheet, most common defaults; user adjustable) | Low 19.5%, high 23.5% | — | Low 35 ppm, high 200 ppm | Low 10 ppm, high 15 ppm | SO2 low 5 ppm, high 10 ppm |
| Honeywell BW MicroClip XL / X3 (user manual, sample factory set points "as defined by OSHA"; vary by region) | Low 19.5%, high 23.5% | Low 10% LEL, high 20% LEL | Low 35, high 200; TWA 35, STEL 50 ppm | Low 10, high 15; TWA 10, STEL 15 ppm | — |
| Honeywell BW MAX XT II (user manual, sample factory set points) | Low 19.5%, high 23.5% | Low 10% LEL, high 20% LEL | Low 35, high 200; TWA 35, STEL 50 ppm | Low 10, high 15; TWA 10, STEL 15 ppm | — |
| MSA ALTAIR 4X (operating manual, MSHA version, factory-set thresholds) | Low 19.5%, high 23.0% | Low 0.5% CH4, high 1% CH4 | Low 25, high 100; TWA 25, STEL 100 ppm | Low 10, high 15; TWA 10, STEL 15 ppm | NO2 low 2.5, high 5.0 ppm |
| MSA ALTAIR 5X (operating manual, factory-set thresholds) | Low 19.5%, high 23.0% | Low 0.5% CH4, high 1% CH4 | Low 25, high 100; TWA 25, STEL 100 ppm | Low 10, high 15; TWA 10, STEL 15 ppm; H2S-LC low 5, high 10 | SO2 2.0 / 5.0; NO2 2.0 / 5.0; NH3 25 / 50; Cl2 0.5 / 1.0; HCN 4.5 / 10.0; PH3 0.3 / 1.0; CO2 0.5% / 1.5% vol |
| MSA ALTAIR 2X (bulletin, low and high alarm in ppm by sensor) | — | — | CO 25 / 100; CO-H2 25 / 100 | H2S 10 / 15; H2S-LC 5 / 10 | SO2 2 / 5; NO2 2.5 / 5; NH3 25 / 50; Cl2 0.5 / 1 |
Every manufacturer states that set points are user adjustable and vary by region; the figures above are the printed defaults, not a rule. OSHA and NIOSH limits are in the exposure-limit guides linked below.
Source: Honeywell BW Clip datasheet (Rev A, 07/23), GasAlertMicroClip XT/XL/X3 user manual and BW MaxXT II user manual (129541-L3 Rev G); MSA ALTAIR 4X operating manual (10128754, MSHA version), ALTAIR 5X operating manual (10150945 r02) and ALTAIR 2X bulletin (0802-96-MC).
Questions people ask
What are some highly toxic gases that can be fatal?
By NIOSH IDLH, the level immediately dangerous to life or health: chlorine 10 ppm, nitrogen dioxide 13 ppm, hydrogen cyanide 50 ppm, phosphine 50 ppm, hydrogen sulfide 100 ppm, sulfur dioxide 100 ppm, ammonia 300 ppm, benzene 500 ppm, carbon monoxide 1,200 ppm. The table on this page gives each with its OSHA PEL and the sensor that reads it.
What are the 10 most flammable gases?
By the width of the flammable range and the lowest LEL in the sources on this page: acetylene (2.5 to 100 percent), hydrogen (4.0 to 75.6), hydrogen cyanide (5.6 to 40), methanol (6.0 to 36), hydrogen sulfide (4.0 to 44), methane (5.0 to 15), carbon monoxide (12.5 to 74), propane (2.1 to 9.5), pentane (1.5 to 7.8), hexane (1.1 to 7.5). RKI Instruments’ FAQ notes the most explosive energy is near the midpoint of each range.
What are the flammable gases found in confined spaces?
Methane in sewers, digesters, landfills and manure pits; propane, gasoline and solvent vapor in tanks and pits; hydrogen from batteries and some processes; hydrogen sulfide and carbon monoxide at very high concentrations. Above 10 percent of the LEL the atmosphere is hazardous under 1910.146(b).
What are some common toxic gases found in confined spaces?
Hydrogen sulfide and carbon monoxide first, which is why they are two of the four standard sensors; then carbon dioxide, ammonia, chlorine, sulfur dioxide, nitrogen dioxide and solvent vapors depending on the space. The construction scope note names sewers, digesters, lift stations, boilers and manholes among the spaces.
What are three toxic gases that are present in confined spaces?
Hydrogen sulfide (OSHA ceiling 20 ppm), carbon monoxide (PEL 50 ppm) and carbon dioxide (PEL 5,000 ppm) are the three met most often; a four-gas monitor reads the first two and the oxygen channel shows displacement by the third.
What is the deadliest gas to humans?
Of the gases on this page, chlorine and nitrogen dioxide have the lowest IDLH values (10 and 13 ppm), hydrogen cyanide and phosphine 50 ppm, hydrogen sulfide 100 ppm; hydrogen sulfide kills most often at work because it is met most often and numbs the sense of smell at the dangerous level.
What are examples of hazardous gases?
The 17 in the table: hydrogen sulfide, carbon monoxide, carbon dioxide, ammonia, chlorine, sulfur dioxide, nitrogen dioxide, hydrogen cyanide, phosphine, benzene, toluene, xylene, hexane, methanol, methane, propane and hydrogen, each with where it is met and the sensor that reads it.
Is methane highly flammable?
Yes: 5.0 to 15.0 percent by volume in air by RKI’s chart, and natural gas is mostly methane. It has no OSHA exposure limit because it does not poison; it burns and displaces oxygen, so the LEL and oxygen channels read it.
Is hydrogen gas toxic to humans?
No; it is a simple asphyxiant with no exposure limit. Its hazard is fire: a 4.0 to 75.6 percent flammable range, the widest of the common gases, read on a catalytic LEL sensor because an infrared sensor cannot see it (RKI’s FAQ).
What device detects toxic gas?
An electrochemical sensor for each toxic gas (H2S, CO, SO2, NO2, HCN, NH3, PH3, Cl2) in a single-gas or multi-gas monitor, a PID for solvent vapors and benzene, and detector tubes for gases with no sensor; the band lists the monitors by gas.
Which gas detector should be used before hot work in confined spaces?
A pumped four-gas monitor with a probe to read oxygen and LEL at the bottom of the tank or pit before the arc, then a diffusion four-gas unit on the welder for oxygen and CO; the welding guide on this site covers the 1910.252 rules.
Related guides
What is H2S, and why does it kill?Where it comes from and what it doesCarbon monoxide exposure limitOSHA 50 ppm, NIOSH 35 ppmCarbon dioxide exposure limitsThe PEL, STEL and IDLH, and CO2 monitorsLEL and UEL by gasMethane, propane, hydrogenConfined space gas testing orderOxygen, then flammables, then toxicsWhat is a gas monitor?Detector, monitor or meter, and what triggers oneThe ratings, test results and standards shown here are the manufacturers' published values, reproduced as published. Nothing on this page is a recommendation by Bayshore Safety that any product is suitable for a particular task, hazard or workplace. Selecting safety products and protective equipment is the responsibility of the end user and the employer, based on their own hazard assessment and the regulations that apply to their work. Confirm every value on the manufacturer's own documentation before use.