# List of highly toxic gases

Many gases have toxic properties, which are often assessed using the LC$_{50}$ (median lethal concentration) measure. In the United States, many of these gases have been assigned an NFPA 704 health rating of 4 (may be fatal) or 3 (may cause serious or permanent injury), and/or exposure limits (TLV, TWA/PEL, STEL, or REL) determined by the ACGIH professional association. Some, but by no means all, toxic gases are detectable by odor, which can serve as a warning. Among the best known toxic gases are carbon monoxide, chlorine, nitrogen dioxide and phosgene.

## Definitions

* Toxic: a chemical that has a median lethal concentration (LC$_{50}$) in air of more than 200 parts per million (ppm) but not more than 2,000 parts per million by volume of gas or vapor, or more than 2 milligrams per liter but not more than 20 milligrams per liter of mist, fume or dust, when administered by continuous inhalation for 1 hour (or less if death occurs within 1 hour) to albino rats weighing between 200 and 300 grams each.
* Highly Toxic: a gas that has a LC$_{50}$ in air of 200 ppm or less.
* NFPA 704: Materials that, under emergency conditions, can cause serious or permanent injury are given a Health Hazard rating of 3. Their acute inhalation toxicity corresponds to those vapors or gases having LC$_{50}$ values greater than 1,000 ppm but less than or equal to 3,000 ppm. Materials that, under emergency conditions, can be lethal are given a Health Hazard rating of 4. Their acute inhalation toxicity corresponds to those vapors or gases having LC$_{50}$ values less than or equal to 1,000 ppm.

## List

| Chemical name | Chemical formula | CAS number | NIOSH IDLH (ppm) | OSHA PEL / NIOSH REL / ACGIH TLV | NFPA 704 Ratings | NFPA 704 Ratings | LC$_{50}$ (ppm) | Odor threshold (ppm) |
| Chemical name | Chemical formula | CAS number | NIOSH IDLH (ppm) | OSHA PEL / NIOSH REL / ACGIH TLV | Health | Special Hazard | LC$_{50}$ (ppm) | Odor threshold (ppm) |
| Arsenic pentafluoride | AsF$_5$ | 7784-36-3 | 5 | NIOSH PEL TWA 0.001 ppm; REL Ceiling 0.0002 ppm (15 min) | 4 | | | |
| Arsine | AsH$_3$ | 7784-42-1 | 3 | NIOSH REL (C) (15 min): 0.002 mg/m$^3$; OSHA PEL: 0.05 ppm; ACGIH (2006) TWA-TLV 0.05 ppm | 4 | | 120 (rat, 10 min) | |
| Bis(trifluoromethyl)peroxide | C$_2$F$_6$O$_2$ | 927-84-4 | 10 | | | | | |
| Boron tribromide | BBr$_3$ | 10294-33-4 | 50 | ACGIH (1990) / PEL Ceiling limit 1 ppm; AEGL-1: 0.33 ppm, AEGL-2: 1.7 ppm, AEGL-3: 5 ppm | 3 | W | | |
| Boron trichloride | BCl$_3$ | 10294-34-5 | 25 | No exposure limits have established; 2500 (TQ) | 4 | W | 2541 (rat, 1 h) | |
| Boron trifluoride | BF$_3$ | 7637-07-2 | 25 | OSHA, ACGIH (1962) 1 ppm ceiling | 4 | | 436 (4 h) | |
| Bromine | Br$_2$ | 7726-95-6 | 3 | OSHA PEL 8-hr TWA: 0.1 ppm; ACGIH (1991) STEL-TLV 0.3 ppm | 3 | OX | 174 (mice) | <0.01 |
| Bromine chloride | BrCl | 13863-41-7 | | 1500 (TQ) | 3 | OX | | |
| Bromomethane | CH$_3$Br | 74-83-9 | 250 | NIOSH PEL 20 ppm | 3 | | 302 (Rat, 8 h) | |
| Carbon monoxide | CO | 630-08-0 | 1,200 (moderately toxic) | ACGIH (1989) TWA TLV 25 ppm; NIOSH 35 ppm; NIOSH 200 ppm Ceiling limit | 3 | | 4600/5000 | |
| Chlorine | Cl$_2$ | 7782-50-5 | 10 | ACGIH (1986) STEL-TLV 1 ppm | 3 | OX | 433 (10 min), 250 (30 min) | 1 |
| Chlorine pentafluoride | ClF$_5$ | 13637-63-3 | | AEGL-2: 0.33 ppm, AEGL-3: 2.7 ppm | | | 194 (Rat) | |
| Chlorine trifluoride | ClF$_3$ | 7790-91-2 | 12 | ACGIH (1979) Ceiling limit 0.1 ppm | 4 | W+OX | 95 (Rat, 4 h) | Inadequate odor |
| Chloropicrin | CCl$_3$NO$_2$ | 76-06-2 | 2 | ACGIH (1990) TWA-TLV 0.1 ppm | 4 | | 9.7 (Mouse, 4 h) | |
| Cyanogen | C$_2$N$_2$ | 460-19-5 | | ACGIH (1966) TWA-TLV 10 ppm | 4 | | | |
| Cyanogen chloride | CNCl | 506-77-4 | | ACGIH (1977) ceiling limit 0.3 ppm | 4 | | | |
| Diazomethane | CH$_2$N$_2$ | 334-88-3 | 2 | ACGIH (1970) TWA-TLV 0.2 ppm | 4 | | 175 (Cat, 10 min) | |
| Diborane | B$_2$H$_6$ | 19287-45-7 | 15 | ACGIH (1990) TWA-TLV 0.1 ppm | 4 | W | 40 (Rat, 4 h) | Inadequate odor |
| Dichloroacetylene | C$_2$Cl$_2$ | 7572-29-4 | | NIOSH REL 0.1 ppm Ceiling | | | | |
| Dichlorosilane | H$_2$Cl$_2$Si | 4109-96-0 | | 2500 (TQ) | 4 | W | 1785-2092 | |
| Dimethylmercury | HgC$_2$H$_6$ | 593-74-8 | | | 4 | | | |
| Ethylene oxide (anhydrous) | C$_2$H$_4$O | 75-21-8 | 800 | REL TWA <0.1 ppm, 5 ppm Ceiling | 3 | | 90 mg/L (fish, 24 h) | |
| Fluorine | F$_2$ | 7782-41-4 | | ACGIH (1970) STEL-TLV 2 ppm | 4 | OX | | 20 ppb |
| Formaldehyde (anhydrous) | CH$_2$O | 50-00-0 | 20 | NIOSH REL 0.016 ppm, Ceiling limit 0.1 ppm | 3 | | 333 (mouse, 2 h) | |
| Germane | GeH$_4$ | 7782-65-2 | | NIOSH REL 0.2 ppm | 4 | | 440 (mice 2 h) | |
| Hydrogen azide | HN$_3$ | 7782-79-8 | | NIOSH REL: 0.1 ppm Ceiling | 4 | | | |
| Hydrogen cyanide | HCN | 74-90-8 | 50 | PEL TWA 10 ppm; REL ST 4.7 ppm | 4 | | 503 (Rat, 5 min) | |
| Hydrogen fluoride | HF | 7664-39-3 | 30 | PEL TWA 3 ppm; REL TWA 3 ppm (2.5 mg/m³); TWA 3 ppm | 4 | POI | LC$_{Lo}$ 313 (rabbit, 7 hr) | |
| Hydrogen selenide | H$_2$Se | 7783-07-5 | 1 | ACGIH (1990) TWA-TLV 0.05 ppm | 4 | | 1.8 (G.Pig, 47 h) | |
| Hydrogen sulfide | H$_2$S | 7783-06-4 | 100 | NIOSH PEL: Ceiling 20 ppm; REL Ceiling 10 ppm; ACGIH (1990) STEL-TLV 15 ppm | 4 | | 444 | 0.3 |
| Hydrogen telluride | H$_2$Te | 7783-09-7 | | | 4 | | | |
| Nickel tetracarbonyl | Ni(CO)$_4$ | 13463-39-3 | 2 | NIOSH TWA PEL 0.001 ppm; ACGIH (1980) TWA-TLV 0.05 ppm | 4 | | 9.642 | |
| Nitrogen dioxide | NO$_2$ | 10102-44-0 | 13 | NIOSH PEL Ceiling 5 ppm; REL ST 1 ppm | 3 | OX | 99 (mice, 1 h) | 4 |
| Osmium tetroxide | OsO$_4$ | 20816-12-0 | 0.001 | NIOSH PEL TWA 0.0002 ppm; REL TWA 0.0002 ppm, ST 0.0006 ppm | 3 | OX | 40 (rat, 4 h) | 0.0019 |
| Oxygen difluoride | OF$_2$ | 7783-41-7 | 0.5 | NIOSH PEL TWA 0.05 ppm; REL Ceiling 0.05 ppm; ACGIH (1983) ceiling limit 0.05 ppm | 4 | OX | 26.067 (Rhesus monkeys, 1 h) | 0.5 |
| Perchloryl fluoride | ClFO$_3$ | 7616-94-6 | 100 | NIOSH PEL TWA 3 ppm; REL TWA 3 ppm, ST 6 ppm; ACGIH (1962) STEL-TLV 6 ppm | 3 | OX | 385 (Rat, 4 h) | |
| Perfluoroisobutylene | C$_4$F$_8$ | 382-21-8 | | 1.2 ppm | | | 17 (Rat, 10 min) | |
| Phosgene | CCl$_2$O | 75-44-5 | 2 | NIOSH PEL TWA 0.1 ppm; REL TWA 0.1 ppm, Ceiling 0.2 ppm; ACGIH (1992) TWA-TLV 0.1 ppm | 4 | | | 0.5 to 1.5 |
| Phosphine | PH$_3$ | 7803-51-2 | 50 | NIOSH 0.3 ppm time weighted average; ACGIH (1992) STEL-TLV 1 ppm | 4 | | 0.44 (Rat, 4 h) | |
| Phosphorus pentafluoride | PF$_5$ | 7647-19-0 | 25 | TLV - TWA 0.25 ppm ACGIH & OSHA | 4 | | | |
| Selenium hexafluoride | SeF$_6$ | 7783-79-1 | 2 | OSHA PEL, NIOSH REL, ACGIH (1992) TWA-TLV 0.05 ppm | 3 | | | |
| Silicon tetrachloride | SiCl$_4$ | 10026-04-7 | | OSHA PEL, NIOSH REL 5 ppm; ACGIH TLV 2 ppm | 3 | W | 8000 (Rat, 4 h) | 1 - 5 |
| Silicon tetrafluoride | SiF$_4$ | 7783-61-1 | | | 3 | W | 922 (Rat, 1 h) | |
| Stibine | H$_3$Sb | 7803-52-3 | 5 | OSHA PEL/NIOSH REL/ACGIH TLV: 0.1 ppm | 4 | | | |
| Disulfur decafluoride | S$_2$F$_{10}$ | 5714-22-7 | 1 | ACGIH (1962) ceiling limit 0.01 ppm | 4 | OX | 2 (Rat, 10 min) | |
| Sulfur tetrafluoride | SF$_4$ | 7783-60-0 | | OSHA PEL/NIOSH REL/ACGIH (1992) ceiling limit 0.1 ppm | 3 | W | 40 (Rat, 1 h) | |
| Tellurium hexafluoride | TeF$_6$ | 7783-80-4 | 1 | NIOSH REL/ACGIH (1992) TWA-TLV 0.02 ppm | | | | |
| Tetraethyl pyrophosphate | C$_8$H$_{20}$O$_7$P$_2$ | 107-49-3 | 5 | ACGIH (2006) TWA-TLV 0.01 mg/m$^3$ | 4 | | | |
| Sulfotep | C$_8$H$_{20}$O$_5$P$_2$S$_2$ | 3689-24-5 | 10 | NIOSH PEL TWA 0.2 ppm | 4 | | 38 (Rat, 4 h) | |
| Trifluoroacetyl chloride | C$_2$ClF$_3$O | 354-32-5 | | | | | | |
| Tungsten hexafluoride | WF$_6$ | 7783-82-6 | | OSHA PEL TWA 2.5 ppm; ACGIH TLV TWA 3 ppm | 3 | W | 218 (Rat, 1 h) | |