Pharmaceutical aerosols use propellants to create the pressure necessary for dispensing the product in the form of fine particles or mist. There are several types of propellants used in pharmaceutical aerosols, categorized into three main groups:

1. Liquefied Gas Propellants

These propellants are in liquid form under pressure but turn into gas when released. They are widely used due to their ability to vaporize and create the desired spray effect.

  • Chlorofluorocarbons (CFCs): Previously popular due to their stability and effectiveness, but largely phased out due to their harmful environmental impact on the ozone layer.
  • Hydrofluorocarbons (HFCs): These have replaced CFCs in many applications as they do not harm the ozone layer. Common HFCs used include HFC-134a and HFC-227a.
  • Hydrocarbons: These include propane, butane, and isobutane. They are inexpensive and have good propellant properties, but they are flammable, which can be a safety concern in some applications.

2. Compressed Gas Propellants

These are gases compressed into the aerosol container, which provides pressure to expel the product.

  • Nitrogen (N₂): Commonly used for non-flammable and non-reactive applications.
  • Carbon dioxide (CO₂): Used for products that require a steady expulsion rate; it is non-flammable and inexpensive.
  • Nitrous oxide (N₂O): Less commonly used but effective for delivering a consistent spray.

3. Mixed Systems

In some cases, a combination of liquefied gas and compressed gas propellants is used to achieve the desired performance characteristics of the aerosol, such as a specific spray pattern or pressure release.

Each type of propellant has its own advantages and is chosen based on factors such as safety, environmental impact, compatibility with the product, and desired aerosol characteristics.