The Pharmaceutics and Compounding Laboratory
Aerosols

Aerosols are unique among the pharmaceutical dosage forms because they depend on the function of a container, its valve assembly, and propellants for the physical delivery of the ingredients. The aerosol container is referred to as a pressurized package. The pressure inside the package is created by the presence of one or more liquefied or gaseous propellants. When the valve is actuated, the pressure forces the contents of the package out through the opening in the valve. The physical form of the expelled contents is a function of the product formulation and the type of valve employed.

aerosol

Aerosols used to provide an airborne mist are called space sprays and include room disinfectants and deodorizers. This group of aerosols produce particles that are usually less than 50 μm in size. This will ensure that the dispersed droplets or particles will remain airborne for a prolonged period of time. A one-second burst from a typical space spray will produce 120 million particles, of which a substantial number will remain airborne for an hour.

Aerosols intended to carry the active drug to a surface are called surface sprays or surface coating sprays. This class of sprays includes products such as deodorant sprays, hair sprays, perfume and cologne sprays, shaving lathers, paint sprays, and various household products such as spray starch, waxes, polishes, and cleaners.

Comparison of Space Sprays, Surface Sprays, and Foam Aerosols
Property Space Sprays Suface Sprays Foam Aerosols
Product Concentrate (%) 2 - 20 20 - 75  
Propellant (%) 70 - 98 25 - 80 6 - 10
Pressure (psig at 70°F) 30 - 40 25 - 55 35 - 55
Particles (μm) <1 - 50 50 - 200  

Pharmaceutical aerosols emit liquid or solid materials in a gaseous medium when they are actuated. The contents may be a fine mist, a course wet or dry spray, a steady stream, or a stable or fast-breaking foam. Pharmaceutical aerosols are intended to deliver active drugs for inhalation, nasal, buccal, and sublingual administration. Aerosols are also available for topical, rectal, and vaginal administration.

Some general advantages of pharmaceutical aerosols include:

    1. Aerosols are easy to use. Medication is dispensed at the push of a button. No ancillary equipment is needed.
    2. Aerosol application is a clean process which requires minimal patient cleanup after using the product.
    3. A portion of medication may be easily withdrawn without contaminating the remaining material. If the product is sterile, sterility can be maintained throughout the product's shelf life.
    4. The active drug is protected from oxygen and moisture. The usual aerosol container is opaque, which also protects the drug from light.
    5. By proper formulation and valve control, the physical form and the particle size of the emitted product may be controlled.
    6. If the dosage must be regulated, a metered dose valve can be used which will control the accuracy of the administered dose.

Many pharmaceutical aerosols are used for oral (i.e., buccal and sublingual), nasal, or inhalation administration of vaccines, antiviral compounds, and hormones. These aerosols provide the advantage of a rapid onset of action and avoid the first pass effect and gastrointestinal tract degradation. In some cases, lower drug dosages can be used, which have the additional benefit of minimizing adverse reactions. Using these routes also provides a viable alternative for administration of drugs that exhibit erratic pharmacokinetics after oral or parenteral administration.

Other pharmaceutical aerosols are used for topical, vaginal, and rectal administration. Topical medications can be applied as a spray, foam, or semisolid in a uniform thin layer without having to touch or mechanically irritate the affected area. The use of an aerosol will also limit the potential for overuse of a product compared to lotions, creams, and ointments. For vaginal and rectal applications, aerosols can be delivered as an expanding foam to ensure a direct and extensive contact between the drug and the mucosa.