A lifting gas, or a lighter-than-air gas, is a gas that has a lower density than typical atmospheric gases. As a result, lifting gases will naturally rise above them. Lifting gases are utilized by lighter-than-air aircraft (such as hot air balloons, airships, and blimps) to create buoyancy and lift for flight.

The density of air at standard conditions for temperature and pressure is 1.28 grams per liter, so all lifting gases are less dense than this.

Most modern lighter-than-air aircraft utilize helium or hot air.

Helium

Helium is the most common lifting gas used today. Helium is noncombustible and is the second lightest gas making it extremely suitable for lighter-than-air flight. Helium does present some disadvantages such as:

  • Cost: Helium is much more expensive than other common lifting gases.
  • Scarcity on Earth: Helium is abundant in the universe but scarce on Earth. Helium is functionally a non-renewable resource.
  • Diffusion: Helium (and hydrogen) molecules are extremely small and diffuse through common materials such as latex. As a result, lighter-than-air aircraft are typically constructed with Mylar/BoPET.

Hydrogen

Hydrogen is the lightest gas in existence. Additionally, it is universally abundant and can be harvested in high quantities. In the early days of lighter-than-air flight, hydrogen usage was widespread, but the chemical has since fallen out of favor as it is extremely flammable and presents many safety risks. Hydrogen has been at the core of several airship-related accidents, including the Hindenburg disaster.

Many countries have banned the usage of hydrogen for lighter-than-air flight, although some allow it for fringe use in recreational ballooning.

Hot Air

Applying heat to a volume of air will cause it to expand, become less dense, and consequently rise. Hot air is used primarily in recreational ballooning. The average temperature of air in hot air balloons is 212 °F, or 100 °C.

Ammonia

Ammonia has seen occasional use as a lifting gas in ballooning. The compound (made of nitrogen and hydrogen) is inexpensive but relatively heavy. Ammonia is also poisonous and an irritant to life systems, so it is unsuitable for most cases of flight.

Methane

Methane is the main component of natural gas and is applicable for occasional use as a lifting agent. Methane molecules are larger than hydrogen and helium and bring the benefit of leaking slower than them. Unfortunately, methane is a very flammable greenhouse gas, preventing the chemical compound from seeing widespread use.

Other theoretically usable lifting gases

To generate lift, a gas only needs to be less dense than standard air condition dry air, making many substances and chemicals potential lifting gases. Gases like water vapor, hydrogen fluoride, acetylene, neon, and nitrogen are theoretically capable for lighter-than-air flight. However, a lifting gas must also be safe for human use (nonflammable, nontoxic), inexpensive, and abundant to see widespread use in aviation.