Which are the lightest non-flammable gases

Which gases are lighter than air?

There are only a limited number of gases that are lighter than air; H. their molecular weight is less than 28.836 g / mol. These are the following 11 gases, 7 of which are flammable:

Ammonia has a special position. Due to its flame resistance and high LEL, ammonia is classified as "non-flammable" outdoors and when transporting dangerous goods. There is therefore no risk of explosion in these two cases, but it is a breath poison. In addition, it combines with the air humidity and is therefore heavier than air if it leaks in the open air.

In the case of hydrogen fluoride, normal room temperatures (HF)3-Complexes are formed, making the resulting gas heavier than air.

Of these 11 gases, the fire brigade is likely to encounter methane in the form of natural gas in NBC operations. Natural gas consists of 75 - 99% methane and a (natural) gas leak in buildings or lines in the open is one of the standard operations of the fire brigade. In addition, natural gas is being used more and more as a fuel in vehicles.

Biogas is becoming more and more important. Biogas (which, in addition to the biogas plant, can also be used as digester gas, e.g. in the sewage treatment plant or sewers) is a mixture of 45 - 75% methane (CH4), 25 - 55% carbon dioxide (CO2), 0 - 5% nitrogen (N2), 0 - 2% oxygen (O2), 0 - 1% hydrogen sulfide (H2S) and traces of other gases. Depending on its composition, biogas can be lighter, heavier or just as heavy as air. The system operator may be able to provide more precise information on the gas composition and behavior. Normally, biogas tends to separate after being released. Substances that are lighter than air float upwards, the other substances spread out on the ground.

All other explosive gases and gas mixtures are heavier than air.

With the exception of flammable hydrogen cyanide (HCN, M = 27 g / mol), the following applies: Flammable vapors are always heavier than air!

The spread of gases depends on the relative gas density and the air currents. What is the relationship between the molecular weight and the relative gas density?

The gas law applies: mass / volume = molar mass * [pressure / (gas constant R * temperature)]

The density of a gas is therefore proportional to the molar mass at the same temperature and pressure, i.e. for substances with a molar mass less than air (28.836 g / mol), the relative gas density is automatically less than one under the same ambient conditions


- Leaflet T 055 (BGI / GUV-I 8617), 07/2009 Measuring and warning devices for explosion protection, BG Chemie

- Rist D .: Dynamics of real gases, Springer Verlag Berlin

- Biogas handbook Bavaria

- Book http://www.abc-gefahren.de/blog/2013/09/30/buch-standard- Einsatz-regel-bei-photovoltaik-windenergie-und-biogasanlagen/

Author: Markus Held

is an environmental and safety engineer (M.Sc.), district trainer for the fire brigade in NBC operations and NBC specialist advisor in fire and disaster control. His focus is on security measures. He is pleased to be able to support Andre Schild in his commitment to a better level of education in the ABC area.


  1. Dear Mr. Held,

    please correct your statement on the segregation of biogas. Both in T 055 and in TRGS 529 it is clearly proven that biogas does not segregate due to gravity.

  2. Dear Mr. Eßer-Luber,

    I am familiar with specialist literature such as T055 and TRGS 529, which do not specify any segregation of biogas, as well as specialist literature such as the book "Standard-Einsatz-Regel Einsatz bei ... biogas plants", publications by the BayLfU and the LFS Baden-Württemberg, one of them Describe the segregation of the biogas when it is released into the atmosphere. I share the opinion of a chemistry professor I know who said: "Sooner or later the biogas will segregate after being released into the atmosphere, only the timing of the segregation is difficult to assess". Incidentally, among the professionally reputable literature sources known to me, the number of sources that indicate segregation predominates. In this respect, I do not see any need for correction with regard to my statement on biogas, as this corresponds to my assessment.

  3. Dear Sir or Madam, My question is, are nitrogen oxides, as they escape from the diesel engine, heavier or lighter than the ambient air on our roads?
    I would be very grateful for a reply.
    With best regards
    Hubert Weimann

  4. I am so happy to read this. This is the kind of manual that needs to be given and not the accidental misinformation that’s at the other blogs. Appreciate your sharing this greatest doc.

  5. Nitrogen oxides are heavier than air and have a lifespan of approx. 1 day near the ground. Since the exhaust gases come out of the exhaust pipe at very high temperatures, like all hot gases they first rise to the top before they sink back to the ground as they cool down and are broken down there.

  6. I not sure where you're getting your info, but great topic. I needs to spend some time learning more or understanding more. Thanks for great info I was looking for this info for my mission.

  7. Good day.

    I just noticed your listing of gases lighter than air.

    This table suggests to me that the "light" gases are not in depressions, shafts or the like. accumulate, since they are "lighter" and would rise accordingly.

    You justify this with the molecular weight, as far as I know with regard to the distribution of gases in the air, the density of the gases is decisive, since the “molecular weight” does not take into account the actual volume in the respective state of aggregation.

    As far as I understand, I would transfer your table so that e.g. B. Oxygen (31.9988 g / mol) would be heavier than water (18.01528 g / mol) and would therefore have to sink in the water. But, as is well known, it doesn't do that in its gaseous state, or is there a fundamental misunderstanding?


    C. Small

  8. Hello Mr. Klein,

    As described in the article, the density of a gas is proportional to the molar mass at the same temperature and pressure (see ideal gas equation), i.e. for substances with a molar mass less than air (28.836 g / mol), the relative gas density is under the same ambient conditions automatically less than one. When comparing gases, a look at the molar mass is sufficient (this is often easier to find out than the density of a gas) to estimate heavier or lighter than air (and in the case of hydrogen fluoride and ammonia you have to know how the substances interact with the ambient air respond;).

    Gases rise from liquids because they have a much lower density than water. In the case of different states of aggregation, the comparison of the molar mass is no longer sufficient, since the ideal gas equation no longer applies here. For the reaction of gases with liquids, the water solubility under the ambient conditions found would be the relevant criterion for assessing possible effects.