The nasal cavity is the first line of defense against airborne pathogens and particles such as pollutants, dust, aeroallergens, and infectious agents. Trapped into the mucosal layer, the inhaled agents are transported towards the pharynx and swallowed via the process of mucociliary clearance which is characterized by the rhythmic beating of cilia in cells lining the surface epithelium of the airways [1]. However, lack of appropriate clearance due to barrier dysfunction and pro-inflammatory responses induced by inhaled agents can disrupt normal functioning of the nose, resulting in bothersome nasal symptoms [2].

Nasal irrigation is a simple and effective technique used to cleanse and moisturize the nasal passages. It typically involves rinsing the nasal cavity with a saline solution, using a spray, squeeze bottle, a neti pot or bulb syringe. This process helps flush out mucus, allergens, infectious agents and pollutants, reducing congestion and alleviating nasal symptoms [3,4].

Nasal irrigation is typically performed with two types of solutions [3,4]:

1. Isotonic saline solutions: these solutions are based on buffered or non-buffered solutions comprising approximately 0.9% NaCl. Isotonic solutions may also include seawater solutions diluted to a final concentration of 0.9% NaCl (undiluted seawater is about 3.1-3.5% NaCl) or iso-osmolar solutions i.e., seawater solutions with a final concentration 0.9% NaCl generated by electrodialysis to remove excess NaCl present in seawater (while maintaining other seawater ingredients intact).
2. Hypertonic saline solutions: these solutions are similar to isotonic solutions but comprise a NaCl content superior to 0.9%, typically between 2-3% NaCl. As above, these solutions can be generated either by dilution of seawater or via selective electrodialysis of seawater to the desired % NaCl in the final solution.

Comparing the efficacy of different solutions, several clinical studies and/or meta-analyses have pointed to a functional superiority of hypertonic versus isotonic solutions in various conditions including rhinitis/rhinosinusitis or after endoscopic sinus surgery (ESS) [5-9]. This superiority is attributed to both osmotic decongestion and mechanical wash properties of hypertonic solutions as a result of the high ionic strength and the cleansing force of the irrigation solution, respectively. On the contrary, isotonic solutions only bear mechanical wash properties [3,4]. From a safety perspective, only minor side effects have been reported when using both solutions (e.g., irritation, mild bleeding, etc.). However, there is tendency for an increased incidence of minor side effects of hypertonic solutions when their hypertonicity exceeds 3% NaCl [5].

Among different hypertonic solutions used in practice, hypertonic seawater solutions of 2.3% NaCl (2.3%HSS), are among the best validated irrigation solutions as based on their efficacy and safety profile revealed in several clinical trials. Below, we will summarize the clinical findings on these solutions.

Healthy subjects
Two clinical trials conducted in healthy subjects have shown that 2.3%HSS increased mucociliary clearance and improved nasal function [10,11]. It was further shown that 2.3%HSS was more effective than isotonic solutions in healthy volunteers [10]. 2.3%HSS thereby offers an effective treatment approach to improve mucociliary clearance, facilitating removal of accumulated mucus, crusts, and entrapped particles and pathogens.