Salt therapy, which can be also referred as halotherapy (Greek: halos – salt, therapy - treatment), is the mode of treatment in a controlled air medium that simulates a natural salt cave microclimate, has historically evolved from the speleotherapy.

Speleotherapy (Greek: speleon – cave, therapy - treatment) is the use of subterranean salt caves as a therapeutic measure in the treatment of chronic airway diseases. Speleotherapy has been acknowledged as a highly effective treatment method for a long time; Polish physician F Bochkowsky has published the first book about therapeutic effects of salt caves' environment on respiratory diseases “About effects of salt dust” already in 1843. The natural dry sodium chloride aerosol is the major curative factor of the cave microclimate. It is formed by the convective diffusion from salt walls. Other factors such as comfortable temperature and humidity regime, the hypobacterial and allergen-free air environment saturated with salt ions enhance the therapeutic effect.
Salt therapy is the method of aerosol therapy, which takes from the speleotherapy the main healing factor – a curative breathing environment, which is aero-dispersed environment saturated with dry sodium chloride aerosol at a specified and controlled mass concentration with a particle size of 0.5-10μm. Salt therapy is performed in a specialized facilities (often decorated with salt), where sodium chloride aerosol is produced only by a generator.

Aerosol therapy is the delivery of aerosolized particles to the pulmonary system for therapeutic purposes. The benefits of such mode of therapy are: rapid onset of agents' action; agent is targeted directly to respiratory system; none or rare systemic side effects; convenient and painless.
Despite the fact that by aerosol therapy very different agents could be administrated, the requirements for the agents’ granulometric properties are the same and are determined by the anatomy of human respiratory tract.

Thus, according to the human airway build-up, treating particles targeted for the lower airways should have diameter below 3μm, for the middle caliber airways 2-6μm and for upper airways 4-10μm. Accordingly, studies have shown that particles with diameter 5-10μm load in oropharynx, larynx and trachea, 2-5μm in bronchi, 0.5-2μm in alveoli and particles below 0.5μm are exhaled (Bates DV et al., Deposition and retention models for internal dosimetry of the human respiratory tract. Task group on lung dynamics. Health Phys. 1966 Feb;12(2):173-207). Unfortunately, up to date used salt aerosol generators produce aerosol, where particles of very different size, predominantly with the diameter of 10μm and above, are present. These generators are not adjustable and based upon granulometric characteristics of the produced aerosol “treat” mainly upper airways, leaving lower respiratory tract uninfluenced. In practice, it means that patient with disorder of lower respiratory tract will get undesired salt therapy for upper airways and virtually no treatment for lower tract. Importantly, administration of unnecessary treatment of any kind can be harmful and should by avoided by all means as the main principle of any kind of therapy is primum non nocere (latin), “first, do no harm”. 
Figure 1. Caliber of human airway compartments

Sodium chloride aerosol improves reological properties of the airway's content and normalizes mucocellular clearance. Sodium chloride is necessary for normal functioning of the bronchial ciliated epithelium, whereas the sodium chloride content in the bronchial secretion of patients with chronic lung diseases decreases. Due to the aerosol's curative influence, the beneficial effects in improving respiratory tract drainage function are evident during salt therapy: relief of sputum expectoration, reduction of sputum viscosity, relief of coughing and positive changes in the auscultator picture of the lungs.

Aerosol of sodium chloride:

Initiates the fluid release into the bronchial lumen, and influences the viscoelastic properties of the bronchial secretion by changing the conformation of protein molecules and releasing water into the outer layers of the clots which promotes evacuation of bronchial sputum (Clarke 1979, Pavia 1978, Wurtemberger 1987). 
  • breaks the ionic bonds within the mucus gel, which reduces the degree of cross linking and entanglements and lower viscosity and elasticity (Ziment 1978) 
  • with chronic infection the mucin macromolecules develop fixed negative charges, causing increased repulsion. The addition of NaCl increases the ionic concentration of the mucus and cause a conformational change by shielding the negative charges and thereby reducing repulsion (Robinson 1995). 
Sodium chloride aerosol has bactericidal and bacteriostatic effects on the respiratory airways microflora and prevents the development of inflammation (Simyonka 1989, Rein 1973). 
  • dehydration of microbial cells
  • adhesion of small particles of salt to microbial bodies. As their mass grows, they precipitate rapidly.
  • beneficial effect on phagocytic activity of alveolar macrophages (Konovalov 1992). 
  • decrease in the index of epitheliocyte infection with pneumococci and the average number of pneumococci per one affected epithelicyte. 
  • decrease in the number of neutrophils and lymphocytes and increase in the number of macrophages in airway mucosa (Voronina at al, 1994).