In any other circumstances, the aerosol droplets can lead to the spread of infections among the people present.

Natural open-air inhalation ‘spaces’ occur in the vicinity of saline graduation towers. These are structures which once served to increase the density of saline waters during the production of table, cooking and therapeutic salts. They lost their significance to the salt manufacturing industry long ago. On the other hand, their role in health resort therapy is flourishing nowadays, as the air around them has been found to be saturated in salt aerosol. This is generated as a result of the saline waters streaming from a height of almost 16 metres and by the action of the wind. The famous saline graduation tower in the Polish spa town of Ciechocinek is 1 714 metres long and stands 15.8 metres high.

Graduation towers are a tourist attraction. However, the quantity and quality of the aerosol in their vicinity is dependent on wind force, temperature and insolation. In a word, they are affected by weather-related factors over which we have no influence.

Given their specific nature and the high ambient humidity within their vicinity, they can be used either in the open air or in a semi-open space. The substance carrying the saline solution is water. With the active substance, NaCl, at only 3%, the body needs to absorb a considerable quantity of the aerosol in order to obtain a therapeutic effect.

Building a saline graduation tower is a massive undertaking because the saline waters must fall from the kind of height already mentioned. As they fall, they are atomised into smaller and smaller particles and it is this process which generates an aerosol with respiratory properties. It is only in this form that an aerosol can be absorbed by the human body, which is equipped with a system for filtering inhaled air. This means that larger particles are retained in the nose, sinuses and bronchi and fail to reach the deeper-lying sections of the respiratory tract.

Nowadays, modern methods are used in order to generate saline mists. The saline solutions can be atomised by means of compressed air in order to generate a fine mist. This method is based on the flow of compressed air through a tube which constitutes part of the nebulising system. Because both the nebulising and the dispersion phases of this aerosol system constitute a liquid, the mist generated is wet and covers the surfaces with droplets. The density of the mist and the size of the droplets generated depend on the air pressure and the speed at which it flows through the outlet nozzle, as well as on the size of the nozzle through which the saline solution is sucked up.

The most effective way of obtaining a saline mist is by using ultrasound.

Research revealed the existence of capillary waves with a high amplitude of oscillations at the ultrasonic focal point. Depending on the density and surface tension of a liquid and the intensity of the ultrasonic energy, these waves trigger a mechanism which splits the particles of the liquid and conveys them into the air in the form of a mist.

Aerosol generated in this way is characterised by density ten times higher than that produced by pneumatic means. The size of the droplets depends on the frequency of the ultrasonic oscillations. In other words, increasing the oscillation frequency reduces the size of the aerosol droplets being generated. A frequency of 2.1 MHz has proved to be the most useful in this respect.

It is not only the ability of the mist to penetrate the respiratory tract which depends on the diameter of the droplets being inhaled, but also its optimal absorption into the mucous membranes. Mists which are very finely atomised, and are thus widely and uniformly dispersed, settle in the deepest-lying sections of the respiratory tract, namely the bronchi, the bronchioles and the alveoli.

At the same time, the speed of the aerosol generation is also dependent on other factors, including the fact that an increase in its viscosity will cause a significant drop in the dispersion speed.

For this reason, using ultrasound to micronise dense solutions and generate aerosols from them is extremely difficult, if not almost impossible on occasion. It is therefore more expedient to disperse liquids of that nature by applying compressed air.

With saline aerosols, coagulation occurs soon after dispersion. In other words, the smaller droplets merge with the larger and will no longer reach the deeper-lying sections of the respiratory tract. The coagulation process means that saline solution is less effective as a therapy than dry salt aerosol.

Saline mists generate increased ambient humidity. This precludes their use in salt caves, salt rooms or any indoor space where salt is used as part of the décor. Salt is hygroscopic. Under conditions of increased relative air humidity, the salt cladding of the walls, for instance, will begin to absorb the moisture from the air and the decoration will start trickling freely to the floor in the form of saline water. Only dry salt aerosol is suitable for use in a décor featuring salt.