The Role of Seawater and Saline Solutions in Treatment of Upper Respiratory Conditions

The history of saline nasal irrigation (SNI) is indeed a long one, beginning from the ancient Ayurvedic practices and gaining a foothold in the west at the beginning of the 20th century. Today, there is a growing number of papers covering the effects of SNI, from in vitro studies to randomized clinical trials and literature overviews. Based on the recommendations of most of the European and American professional associations, seawater, alone or in combination with other preparations, has its place in the treatment of numerous conditions of the upper respiratory tract (URT), primarily chronic (rhino)sinusitis, allergic rhinitis, acute URT infections and postoperative recovery. Additionally, taking into account its multiple mechanisms of action and mounting evidence from recent studies, locally applied seawater preparations may have an important role in the prevention of viral and bacterial infections of the URT. In this review we discuss results published in the past years focusing on seawater preparations and their use in clinical and everyday conditions, since such products provide the benefits of additional ions vs. saline, have an excellent safety profile and are recommended by most professional associations in the field of otorhinolaryngology.


Introduction
The use of water for prophylactic or therapeutic purposes, mostly in the respiratory system, has been known since ancient times. In Yogic practices, different nasal cleansing techniques are used as part of a wider range of body-cleansing procedures. Vedic texts describe several techniques called "neti" [1,2], with "jala neti" [3,4] corresponding to today's concept of nasal cavity irrigation. In the neti techniques, copperware is used for irrigation (to prevent contamination of the solution), the solution is heated to body temperature and an exact salt concentration in the preparation of the solution is specified. This salt content, and consequently, the osmolality of the solution, remains one of the most important parameters in nasal irrigation to the present day.
The osmolality of the commercial compositions of NaCl solution ranges from the physiological level (0.9%) to the hypertonic level with an osmolality of 3% [2]. Solutions with higher osmolality tend to induce side effects, such as nasal burning, blockage and dripping [5]. The osmolality of the solution results not only from NaCl content but also from the other ions contained therein. Besides having effect on osmolality, the ions also show a number of effects on the biology and function of cells and tissues. This is especially important because many commercially available formulations, primarily those based on seawater, contain a number of ions other than Na + and Cl − , and differ significantly from the galenic saline. In this review, the following parameters of these solutions will be discussed:

•
Composition of solution in context of differences between saline and solutions based on seawater. • Mechanism of action in nasal cavity and elsewhere. • Safety and efficacy of use in different indications.

Composition of Saline/Seawater Preparations
Unlike saline, which consists of NaCl dissolved in distilled water, in seawater there are four categories of constituents or solutes: major constituents, minor constituents, trace elements and gases. Average salinity of undiluted seawater is approximately 3.5%, or 35 ppt (parts per thousand). Ninety-nine percent of seawater salinity is due to six major constituents: Cl − , Na + , SO 4 2− , Mg 2+ , Ca 2+ and K + . Salinity is relatively uniform, with ranges of variation of 33-37 ppt in open ocean water, 37-38 ppt in smaller bodies of seawater, such as the Adriatic Sea, and as much as 240 ppt in the Dead Sea. This is why the source of the water in seawater products is such an important factor.
One of the fundamental laws in oceanography, the Forchhammer principle, or the principle of constant proportions, states that the relative proportions of the major constituents of seawater are constant, regardless of different salinities in different sea-water samples [6]. Cl − accounts for 55% of the ions, followed by Na + (30.6%), SO 4 2− (7.7%) and Mg 2+ (4%). Major constituents are also considered to be conservative, i.e., chemically non-reactive and thus stable in oceans and seas over long periods of time. The oceans have a major impact on climate regulation and climate changes, the life cycle of nutrients in nature, energy flow and the biodiversity of its inhabitants. The chemical and physical properties of oceans are changing, with global warming and acidification due to increased carbon absorption. Warmer air can absorb more water than colder air, so as the climate heats up, more water can evaporate into the air, as a result of which salinity increases [7,8] but as mentioned before, different salinities do not affect the relative portions of the major constituents. Besides the major constituents measured in ppt, seawater also contains a number of minor constituents, measured in ppm-parts per million, and trace elements, measured in ppb-parts per billion. However, the principles that apply to the major elements do not apply to the minor and trace elements. This means that many of these elements are biologically or chemically reactive, and that their concentration can be dependent on biological activity and other factors, exhibiting significant local differences. The major constituents of seawater with salinity of 35 ppt at the temperature of 25 • C are shown in Table 1.  69 63 * Seawater sample source. Dittmar (1940): various parts from Atlantic, Indian and Pacific Ocean during the "Challenger" expedition; Cox (1966): locations from the world's oceans (Atlantic, Indian, Pacific). Detail locations of the samples can be seen in Cox (1996) [10]; Riley (1967): Mediterranean, Irish, Baltic Sea; Millero (1996): Baltic Sea, Baltic-North Sea, Red Sea; Štanfel (2006): Adriatic Sea Kvarner Bay.
From Table 1, it is evident that the cations (sodium, potassium, calcium and magnesium) determined in seawater from the Adriatic Sea (Kvarner bay) by the ion-chromatography method are higher at higher levels than cations obtained by various authors mentioned in Table 1.
Another parameter of primary importance for seawater products is osmolality. Seawater with concentrations of approximately 26% to 27% becomes hypertonic (considering the plasma osmolality reference range of 285-295 mOsm/kg [14] and can exert a range of effects associated with hypertonic solutions.

Mechanism of Action
The mechanism of action of nasal irrigation solutions is based on two principles: physical and biological/physiological. The first principle is based on the physical (mechanical) effect of cleansing the nasal mucosa of the accumulated secretion and pathogens. The second principle depends on the effects of the ions on the physiology of the mucosal cells. In the Figure 1 we propose the chain of events following mucosal application of seawater preparations which results in a range of beneficiary effects.  The mechanism displayed in Figure 1 centers on the transport of water through the mucosal epithelial membrane, provoked by the local application of hypertonic solution. The nasal mucosa is hydrated and moisturized by both the local application of solution and the influx of water through the membrane. This leads to increased mucociliary clearance (MCC) [15,16]. Additionally, liquid transport through the membrane results in the accumulation of liquid in the nasal lumen, which concept was also proven in other organs and tissues, both in vitro [17,18] and in vivo [19,20]. Reduction in swelling (oedema) is seen in submucosal tissue, while the immediate effect of excess liquid in the nasal lumen is mechanical cleaning of mucus, crusts and debris. Immediately following this, the state of the mucus changes from gel to sol [21]. The transition of mucus from gel to sol state greatly reduces the amount of energy needed by cilia to transport such mucus [22], significantly improving the efficacy of the mucociliary transport. Additional ionic constituents of seawater show other effects, such as increased cell viability and inflammation reduction ( Figure 1 and Table 2).  [3,20].

Mg 2+
Promotes cell repair and limits inflammation by reducing the eicosanoid metabolism both at the level of the liberation of arachidonic acid and by direct inhibition of the 5-lipoxygenase enzyme. Inhibits exocytosis from permeabilized eosinophils. Reduces apoptosis of respiratory cells.

Ca 2+
Acetylcholine and serotonin act as messengers, increasing calcium intake in ciliated cells and thus regulating ciliary beat frequency and synchronization. Airflow promotes cell calcium intake and ciliary beat via shear-stress-induced mechanotransduction.
K + Anti-inflammatory action. Promotes respiratory epithelium repair via the EGF/EGFR pathway.
Reduces mucous viscosity by acting as a buffer.
Facilitates elimination by ciliary cells movement.
Mucociliary transit time (MTT; the time needed for a compound to be transported a certain distance within the respiratory system) is used to assess the efficacy of mucociliary clearance. Compared to healthy volunteers with mean MTT of 12.01 ± 3.0 min, this time is significantly prolonged in subjects with a history of allergic rhinitis (15.5 ± 3.5 min) and in heavy smokers (16.5 ± 5.0 min) [23]. Similarly, it has been shown that patients with a wide variety of diseases, ranging from septum deviations [24] to chronic sinusitis [25], have prolonged MTT, and that the restoration of mucociliary clearance is of significant importance in treating the disease [26]. Therefore, the efficacy of mucociliary transport might be one of the key mechanisms in the positive effect of nasal irrigation solutions on the nasal tissue [27,28] On the most basic level, MTT depends on the ciliary beat frequency (CBF). Wabnitz et al. used nasal sprays with 0.9% and 3.0% sodium chloride on eight healthy volunteers with a mean baseline CBF of 9.6 Hz. While isotonic saline reduced the CBF firstly to 9.1 Hz (after 5 min) and then to 8.8 Hz (after one hour), use of 3.0% saline increased the CBF to 10.1 Hz before it returned to near-baseline levels (9.2 Hz) at 60 min [29]. Similar results were seen when monitoring another parameter, saccharine clearance time, which decreased from a median of 11.17 min to a median of 6.83 after application of isotonic saline, and one of 7.14 min after application of hypertonic saline [30]. These results, which show a beneficiary effect with hypertonic saline, but a much smaller or completely absent effect with isotonic saline, are confirmed by other authors [31][32][33][34]. The same effects of hypertonic saline were shown for mucociliary clearance in asthmatic patients [35], subjects with cystic fibrosis [36][37][38], children with bronchiolitis [39] and healthy subjects [19]. On the molecular level, this effect of hypertonic saline seems to be based on the upregulation by of CLC-3, a chloride channel that accounts for the transport of chloride ions in numerous tissues and plays a fundamental role in transepithelial salt and water movement [40].
Besides the abovementioned mechanism involved in the physical and osmotic effects of the solution, different ions in seawater have a number of additional effects. These effects are displayed in Table 2.
The abovementioned findings show that, besides the immediate positive effect of the mechanical cleaning of the mucosal surface, there is an additional and potentially more important positive effect exerted through the facilitation of the physiological function of mucociliary transport achieved by a saline solution of adequate osmolality. Additionally, other ions contained in the solution show a wide range of beneficial physiological effects on a cellular level. Table 3 shows the main safety and efficacy conclusions from clinical trials and in-vitro studies performed over more than 20 years. We searched the MEDLINE, Scopus, Web of Science and Cochrane databases to identify studies of interest. The aim was to identify as much (especially clinical) studies as possible. To achieve this, we used a broad search strategy, including only the basic keywords of "seawater" and "saline". For example, a MeSH search syntax was "Seawater"[Mesh] OR "Saline Solution"[Mesh] OR "Saline Solution, Hypertonic" [Mesh]. Because MeSH indexing takes some time, an additional PubMed search with the same keywords was performed for the studies published over the last three years. Additional studies were identified through Scopus, and in particular, by following "Times Cited" links for the Web of Science results. After examining all the identified studies, we focused on those that, in our opinion, contributed most to the understanding of the safety and efficacy aspects of nasal irrigation use in human medicine. Studies with both seawater and saline solutions in a wide range of osmolalities and compositions were covered. The safety and efficacy of these preparations will be shortly discussed here.  no treatment.

Aspects of Saline/Seawater in Human Use
Ten side effects, of which 8 were considered as "not significant" and 2 as significant, but affected subjects were still "highly satisfied" with the treatment.
Subjects treated with nasal saline used statistically significantly less antibiotic treatment compared to control group.  No adverse events in the treatment group.
Statistically significant decrease in use of oral antihistamines in hypertonic saline group. Kim, 2005 [54] In vitro study  [29] In-vitro study 8 healthy adult subjects One application of four sprays of hypertonic (3.0%) saline (one nostril) and isotonic saline (another nostril) in all subjects.
No adverse effects mentioned.
Cell samples from subjects receiving saline solutions. The administration of hypertonic saline results in a significantly faster CBF 5 min (but not 60 min) after administration.    (2 × 0.14 mL) into each nostril.
Three mild AEs were documented in two subjects in the Nasya/Prevalin group (swallowing difficulties, nasal airways obstruction and headache); none related to the application of the investigational product.
There was no difference between the two treatment groups regarding the global assessment of tolerability provided by the investigators or by the subjects.  One adverse event reported (epistaxis).
Over the study period (mean 44 days) statistically significant reductions in 10 out of 12 symptoms was found.
Only for parameters "impairment of taste" and "impairment of food intake" was no significant change in symptoms observed. Nasal discomforts were detected in two patients in hypertonic seawater group and in two patients in the isotonic group.
Significantly lower total symptom score during the 7th, 14th, 21st and 28th days, lower total endoscopic score on the 21st and 28th days, lower nasal obstruction, facial pain/pressure, headache and trouble sleeping, and lower nasal mucosal oedema, nasal secretion and nasal crusting in patients treated by hypertonic seawater. Seawater group was statistically significantly superior in terms of nasal congestion, nasal discharge, sleep quality and appetite, but not for cough and fatigue.

Safety
The safety of preparations based on both saline and seawater has been proven in numerous studies, with subjects ranging from healthy individuals to infants and pregnant women. More than 60 such studies are listed in Table 3, covering the period of last 23 years. General side effects are rare, while serious side effects are virtually non-existent. Moreover, one must take into account the fact that in most of the studies, subjects had at least one additional condition or diagnosis, such as allergic rhinitis, rhinosinusitis, postoperative status, asthma, bronchiolitis, etc. Most of these conditions require additional therapy, which in itself could be the reason for the side effect(s) ascribed to nasal irrigation treatment. In most of the abovementioned studies, adverse events were either not mentioned in the text of the papers, or were not reported by study participants. In cases where adverse events were mentioned, most pertained to a burning feeling in the nose and throat. Some studies report that the incidence of this adverse event is rather high; for instance, a mild burning sensation was reported by a majority (57% [35]) of subjects in a study by Kumar et al. In the same study, moderate burning, as opposed to mild, was much less pronounced, with only 19% subjects reporting this side effect. Furthermore, the intensity of the burning sensation seemed to be correlated to osmolality of the preparation, with hypertonic preparations causing more adverse events. Other studies report similar rates of burning among their participants. Shoseyov et al. [42] describe burning sensations in four (of a total of 34) paediatric subjects with chronic sinusitis, with three taking hypertonic saline and one taking an isotonic preparation (note a similar rate of adverse events between hypertonic and isotonic groups described in the previous study. However, there are studies where this rate is inverse [28]. Other studies mentioning burning sensations as a side effect of nasal irrigation therapy either fall within the incidence boundaries described above [58,72] or discuss burning as a side effect not affecting subjects' participation in the study or the study's outcome [33,57,62,66].

Efficacy
The efficacy of nasal irrigation solutions has been proven in numerous clinical trials and studies, most of which are listed in Tables 3 and 4. Efficacy has been proven in a variety of populations, from pregnant women and children to adults with a wide range of patholog-ical conditions. Given that the attached list of publications speaks for itself, we concentrate here on presenting the essential facts about a few of the most important indications.  Nine studies (740 patients) were included. Hypertonic nasal irrigation brought greater benefits than isotonic treatment in symptom reduction; however, no difference was shown in SNOT-20 improvement. Effects favouring hypertonic solution were greater in patients with rhinitis compared with rhinosinusitis; in patients under the age of 18 years; in saline irrigation using high volume compared with low volume and in saline irrigation with hypertonicity of <3% and hypertonicity of 3-5% compared with hypertonicity of >5%.
No major adverse effects were reported. Saline nasal irrigation is recommended as an adjunct therapy for common colds/rhinosinusitis, chronic sinusitis, allergic rhinitis and after nasal surgery. It appears to be safe and generally well tolerated, even for children. The use of SNI has the potential to reduce the number of antibiotic prescriptions for acute and chronic sinus infections, and improve outcomes for patients.
Regarding the exact posology, the question remains an open one. There are numerous factors affecting the dosage to be administered. Detailed data are laid out in the tables below, with the basic elements to be considered being age (paediatric vs. adult), indication (allergy, sinusitis, postoperative indications or usage in healthy individuals), product concentration (isotonic or hypertonic, up to approximately 3.5%) and methodology of application (drops, spray, nebulizer or irrigation). Therefore, the dosage may range from a few drops of hypertonic solution in children with URTI to extensive lavage with isotonic solution in adult subjects with a variety of indications.

Chronic Sinusitis
By definition, chronic rhinosinusitis (CRS) is an inflammation of the paranasal sinuses seen in several percent of both the paediatric and adult populations [104]. The diagnosis is based on the presence of at least two of four cardinal symptoms for at least 12 weeks, and is confirmed by physical examination and (if necessary) additional radiological methods [105]. Intranasal spray administration of corticosteroids is known to significantly improve symptoms, and a similar consensus exists for nasal saline irrigation. The use of oral antibiotics may be indicated in cases of acute exacerbations of the disease, although this was not corroborated in the recent Cochrane review on this topic [99]. Similar results have been described in children by a group of Russian authors [106].
Papers listed in Tables 3 and 4 strongly confirm these findings. In a paediatric population, Pham et al. [78] showed that 6-week treatment is well tolerated in children and is useful both as a first-line treatment for CRS and as an effective measure reducing the need for surgery. Regarding tonicity, in another paediatric study, hypertonic solution was shown to be comparable to the isotonic variety in terms of safety, although the number of adverse events was higher in the hypertonic group [42].
Evidence of both safety and efficacy is, expectedly, more abundant in adult populations. Subjects treated with nasal saline used fewer antibiotics compared to the control group [47] and hypertonic solution was reported as superior to isotonic solution [67,73,88]. Other hypertonic saline preparations, such as Dead Sea salt, have also been proven as safe and effective in this indication [56]. While various application methods are used (mostly spray vs. low/large volume irrigation [46,60], the safety profile remains highly favourable across the various studies.

Allergic Rhinitis
Allergic rhinitis is an extremely common condition that is also commonly overlooked in the diagnostic process, resulting in significant public health effects. Additionally, although it is not a severe illness, allergic rhinitis can significantly complicate the symptoms, diagnosis and clinical course of other diseases [107]. Nasal irrigation preparations have been shown to be effective [16] and safe [70] as both long-term [83] and short term [74] treatments, and to reduce the need for other commonly used treatment options, such as antihistamines, in children [48,53] and pregnant women [108]. The same was proven for the use of nasal steroids [65,77] and systemic drugs [49].

Other Indications
Besides the two major indications listed above, there are numerous studies in other indications, as well as in vitro studies [54] and studies performed on healthy participants, with the latter serving primarily as the proof of concept for the safety and efficacy of nasal irrigation treatments.
Different methods of saline penetration were tested using the Technetium-99 labelled solution, with douching being the method with best penetration in the maxillary sinus [51]. Positive effects of nasal irrigation were proven in healthy army conscripts [50], adult subjects [18,29,45] and otherwise healthy subjects exposed to wood dust [41,43].
Regarding other indications, positive effects were described in paediatric patients with viral bronchiolitis [59], bronchiolitis in the intensive care unit [86], acute sinusitis [80], acute upper respiratory tract infections [82,85], chronic tonsilitis [61] and cold and influenza [62]. Moreover, daily nasal irrigation in the paediatric stage (especially in children who cannot blow their noses) is a practice that should be encouraged as a good habit, even without underlying pathologic conditions. Similar studies exist regarding adult subjects [55,89], including pregnant women [84]. Studies on the postsurgical beneficiary effects of saline solutions [28,75], retrospective studies [109] and those based on questionnaires and surveys [66,76] seem to confirm all of the above-mentioned effects.

The Place of Saline/Seawater Preparations in the COVID-19 Pandemic
Finally, although it is too early to speculate on whether the use of nasal irrigation solutions has a place in preventing or reducing the symptoms of viral infections [110], a recent publication on people infected with coronavirus [89] suggests that this could be an interesting area of research in the near future. Additionally, there is a growing number of papers on this topic, suggesting the potential positive effects of saline irrigations during the pandemic, both as preventive [111][112][113] and a treatment option [89]. In a recent paper, a multidisciplinary group of Belgian authors [114] proposed a detailed hypothesized mechanism of action of saline in coronavirus infections. The mechanism is similar to that proposed in the present article, including, among other aspects, wetting effects on the local tissue, mucus gelling, and the effects of the increased NaCl concentration on mucosa. Due to the effects described earlier in this paper and elsewhere [115], if used early and as an add-on therapy, locally applied nasal irrigation solutions may represent an interesting and promising remedy for all viral infections, including SARS-CoV-2 [116].

Conclusions
Nasal irrigation solutions show numerous positive effects in clinical use in the upper respiratory tract. These are mainly mechanical (cleaning of the mucosa) and related to osmolality (oedema reduction and moisturizing of the epithelium). In our paper, we presented a comprehensive body of evidence regarding the beneficiary effects of nasal irrigation solutions in general as well as for a wide variety of clinical indications, such as infectious diseases of the upper respiratory tract, allergic rhinitis, postoperative care, etc. All information mentioned above, especially the data in Table 2 [3,20], clearly favours seawater preparations over saline. However, a definitive recommendation can be given only after the careful evaluation of EBM levels for each of the papers discussed. Due to its chemical constituents, such as magnesium, calcium, potassium, bicarbonate and other ions, seawater shows a range of additional chemical effects, from promoting cell repair and reducing inflammation to reducing viscosity of the mucus and increasing ciliary beat frequency. Numerous studies in URT patients, pregnant women, children and elderly individuals show exceptionally good safety profiles for seawater preparations [82,98,108,117]. Side effects are rare, and consist mostly of burning feelings and nasal drainage, with serious adverse events practically non-existent.
To the best of our knowledge, a scientifically proven consensus on the exact mechanism of action of seawater in the human upper respiratory tract does not exist. Therefore, based on a comprehensive literature search, we propose a mechanism of action considering all the different aspects of seawater solution(s), from chemical composition to pH and tonicity. Further studies will be needed to confirm the present findings.