list of highly toxic gases list of highly toxic gases many gases have toxic properties, which are often assessed using the lc50 (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 or stel) 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. definition toxic: a chemical that has a median lethal concentration (lc50) 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 lc50 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 lc50 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 lc50 values less than or equal to 1,000 ppm. list chemical name chemical formula cas number niosh idlh in ppm osha pel / niosh rel / acgih tlv nfpa 704 health rating nfpa 704 special hazard rating additional notes arsenic pentafluoride asf5 7784-36-3 5 niosh pel twa 0.001 ppm; rel ceiling 0.0002 ppm(15-min) 4 arsine ash3 7784-42-1 3 niosh rel (c)(15min): 0.002 mg/m3; osha pel: 0.05 ppm; acgih (2006) twa-tlv 0.05ppm 4 lc50: 120 ppm(rat, 10 min) bis(trifluoromethyl)peroxide c2f6o2 927-84-4 10 boron tribromide bbr3 10294-33-4 50 acgih (1990) / pel ceiling limit 1ppm; aegl - 1: 0.33 ppm, aegl - 2: 1.7 ppm, aegl - 3: 5 ppm 3 w boron trichloride bcl3 10294-34-5 25 no exposure limits have established; 2500 (tq) 4 w lc50: 2541 ppm (rat, 1 h) boron trifluoride bf3 7637-07-2 25 osha, acgih (1962) 1ppm ceiling 4 lc50: 436 ppm (4 h) bromine br2 7726-95-6 3 osha pel 8-hr twa: 0.1 ppm; acgih (1991) stel-tlv 0.3ppm 3 ox lc50: 174 ppm(mice) odor threshold: <0.01 ppm bromine chloride brcl 13863-41-7 1500 (tq) 3 ox bromomethane ch3br 74-83-9 250 niosh pel 20 ppm 3 lc50: 302 ppm (rat, 8 h) carbon monoxide co 630-08-0 1,200(moderately toxic) acgih (1989) twa tlv 25ppm; niosh 35ppm; niosh 200ppm ceiling limit 3 lc50: 4600/5000 ppm chlorine cl2 7782-50-5 10 acgih (1986) stel-tlv 1ppm 3 ox odor threshold 1ppm, lc50: 433(10 min), 250 (30 min) chlorine pentafluoride clf5 13637-63-3 aegl - 2: 0.33 ppm; aegl - 3: 2.7 ppm lc50: 194 ppm(rat) chlorine trifluoride clf3 7790-91-2 12 acgih (1979) ceiling limit 0.1ppm 4 w+ox inadequate odor, lc50: 95 ppm (rat, 4 h) chloropicrin ccl3no2 76-06-2 2 acgih (1990) twa-tlv 0.1ppm 4 lc50: 9.7 ppm (mouse, 4 h) cyanogen c2n2 460-19-5 acgih (1966) twa-tlv 10ppm 4 cyanogen chloride cncl 506-77-4 acgih (1977) ceiling limit 0.3ppm 4 diazomethane ch2n2 334-88-3 2 acgih (1970) twa-tlv 0.2ppm 4 lc50: 175 ppm (cat, 10 min) diborane b2h6 19287-45-7 15 acgih (1990) twa-tlv 0.1ppm 4 w inadequate odor, lc50: 40 ppm (rat, 4 h) dichloroacetylene c2cl2 7572-29-4 niosh rel 0.1 ppm ceiling dichlorosilane h2cl2si 4109-96-0 2500 (tq) 4 w lc50: 1785-2092 ppm dimethylmercury hgc2h6 593-74-8 4 ethylene oxide (anhydrous) c2h4o 75-21-8 800 rel twa<0.1 ppm 5 ppm ceiling 3 lc50: 90 mg/l (fish, 24 h) fluorine f2 7782-41-4 acgih (1970) stel-tlv 2ppm 4 ox odor threshold 20ppb formaldehyde (anhydrous) ch2o 50-00-0 20 niosh rel 0.016 ppm ceiling limit 0.1ppm 3 lc50: 333 ppm(mouse, 2 h) germane geh4 7782-65-2 niosh rel 0.2 ppm 4 lc50: 440 ppm (mice 2 h) hydrogen azide hn3 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 lc50: 503 ppm(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 lclo 313 ppm (rabbit, 7 hr) hydrogen selenide h2se 7783-07-5 1 acgih (1990) twa-tlv 0.05ppm 4 lc50: 1.8 ppm(g.pig,4 h) hydrogen sulfide h2s 7783-06-4 100 niosh pel: ceiling 20 ppm; rel ceiling 10 ppm; acgih (1990) stel-tlv 15ppm 4 lc50: 444 ppm, odor threshold 0.3 ppm hydrogen telluride h2te 7783-09-7 4 nickel tetracarbonyl ni(co)4 13463-39-3 2 niosh twa pel 0.001 ppm; acgih (1980) twa-tlv 0.05ppm 4 lc50: 9.642 ppm nitrogen dioxide no2 10102-44-0 13 niosh pel ceiling 5 ppm; rel st 1 ppm 3 ox odor threshold 4 ppm; lc50: 99 ppm (mice, 1 h) osmium tetroxide oso4 20816-12-0 0.001 niosh pel twa 0.0002 ppm; rel twa 0.0002 ppm, st 0.0006 ppm 3 ox lc50: 40 ppm (rats, 4 h); odor threshold: 0.0019 ppm oxygen difluoride of2 7783-41-7 0.5 niosh pel twa 0.05 ppm; rel ceiling 0.05 ppm; acgih (1983) ceiling limit 0.05ppm 4 ox odor threshold: 0.5 ppm; lc50: 26.067 ppm (rhesus monkeys, 1 h) perchloryl fluoride clfo3 7616-94-6 100 niosh pel twa 3 ppm; rel twa 3 ppm, st 6 ppm; acgih (1962) stel-tlv 6ppm 3 ox lc50: 385 ppm (rat, 4 h) perfluoroisobutylene c4f8 382-21-8 1.2 lc50: 17 ppm (rat, 10 min) phosgene ccl2o 75-44-5 2 niosh pel twa 0.1 ppm; rel twa 0.1 ppm, ceiling 0.2 ppm; acgih (1992) twa-tlv 0.1ppm 4 odor threshold 0.5 to 1.5 ppm phosphine ph3 7803-51-2 50 niosh 0.3ppm time weighted average; acgih (1992) stel-tlv 1ppm 4 lc50: 0.44 ppm (rat, 4 h) phosphorus pentafluoride pf5 7647-19-0 25 tlv - twa 0.25 ppm acgih & osha 4 selenium hexafluoride sef6 7783-79-1 2 osha pel, niosh rel, acgih (1992) twa-tlv 0.05ppm 3 silicon tetrachloride sicl4 10026-04-7 osha pel, niosh rel 5 ppm, acgih tlv 2 ppm 3 w lc50: 8000 ppm (rat, 4 h) odor threshold: 1 - 5 ppm silicon tetrafluoride sif4 7783-61-1 3 w lc50: 922 ppm (rats, 1 h) stibine h3sb 7803-52-3 5 osha pel/niosh rel/acgih tlv: 0.1 ppm 4 disulfur decafluoride s2f10 5714-22-7 1 acgih (1962) ceiling limit 0.01ppm 4 ox lc50: 2 ppm (rat, 10min) sulfur tetrafluoride sf4 7783-60-0 osha pel/niosh rel/acgih (1992) ceiling limit 0.1ppm 3 w lc50: 40 ppm (rat, 1 h) tellurium hexafluoride tef6 7783-80-4 1 niosh rel/acgih (1992) twa-tlv 0.02ppm tetraethyl pyrophosphate c8h20o7p2 107-49-3 5 acgih (2006) twa-tlv 0.01 mg/cubic meter 4 sulfotep c8h20o5p2s2 3689-24-5 10 niosh pel twa 0.2 ppm 4 lc50: 38 ppm (rat, 4h) trifluoroacetyl chloride c2clf3o 354-32-5 tungsten hexafluoride wf6 7783-82-6 osha pel twa 2.5 ppm, acgih tlv twa 3 ppm lc50: 218 ppm (rat, 1 h) notes ^ ^ decomposes in gaseous form. ^ explodes in gaseous form.