Table salt may contain additional unwanted ingredients. Only pure NaCl or minerals found in sea water should go into such a solution. But food industry is allowed to add much more.
Nasal irrigation salt needs just to be NaCl used in isotonic concentration. The indian tradition is based on water from rivers, which is not isotonic and now often contaminated. Clean (sterilised?), potable/drinking tap-water works as well, but appears much harsher than needed. So isotonic solutions are clearly preferable.
One can use regular salt for that. As long as it's pure NaCl. And one may be 'lucky' in using all other forms of table salt. But regular table salt is not just NaCl!
What is neti-salt?
First link to vendor appearing in a websearch:
Ingredients: Pharmaceutical grade sodium chloride (99.99%) USP.
Directions: Place a heaping 1/4 teaspoon in your Neti Pot and add warm water.
Whether sea salts minerals are in there as well is not really that much more beneficial, but when it's regular "table salt" then most jurisdiction allow a lot of other ingredients in it that you do not want in your nose pr on your mucus membrane.
Regular kitchen table salt can be used but is not recommended because of all the additives.
Apart from possible adulterations when using "funny" salts ('Himalaya' etc) allowed anti-caking agents for example are:
341 tricalcium phosphate 460(ii) powdered cellulose 470b magnesium stearate 500 sodium bicarbonate 535 sodium ferrocyanide 536 potassium ferrocyanide 538 calcium ferrocyanide 542 bone phosphate (i.e. Calcium phosphate) 550 sodium silicate 551 silicon dioxide 552 calcium silicate 553a magnesium trisilicate 553b talcum powder 554 sodium aluminosilicate 555 potassium aluminium silicate 556 calcium aluminosilicate 558 bentonite 559 aluminium silicate 570 stearic acid 900 polydimethylsiloxane
Some of these are listed in the Eu as E170, E504, E535, E536, E551, E559…
Table salt is sodium chloride combined with iodine sources (for nutrition), stabilizers for the iodine, and anti-caking compounds to make it pour by preventing it from absorbing water from the air.
So supermarket bought regular table salt can be quite a bit too irritating for a nasal irrigation. Depending on jurisdiction not all ingredients have to be listed on table salt.
Nasal irrigation is based on warm saline solution, i.e. salt dissolved in lukewarm tap water. ... However, chemical anti-caking (aka “free-flowing”) additives used in common table salt (e.g. sodium silicoaluminate) produce an unpleasant burning sensation in the nose and are not advised medically or by Yoga instructors.
–– The problems of salt for neti - NetiNext
anti-caking agents or preservatives (these can be irritating to the nasal lining)
–– SALINE SINUS RINSE RECIPE 2019 American Academy of Allergy, Asthma & Immunology.
With just a little chemistry knowledge anyone can conclude: among other things nasal irrigation is used for reducing mucus viscosity. This is clearly not an outcome achieved if adding bentonite into a solution for the nasal cavity. While listed as non-toxic non-irritating, bentonite is used as cat litter, and hence it does in the nose what it does in the litter box: increases viscosity of mucus. This effect is there for all anti-caking agents. Some are directly irritating the membranes, all are doing the opposite of what one wants to achieve.
Further regular nasal irrigation isn't very well supported for most use case anyway:
Karen Head, Kornkiat Snidvongs, Simon Glew, Glenis Scadding, Anne GM Schilder, Carl Philpott, Claire Hopkins: "Saline irrigation for allergic rhinitis", in: Cochrane Database of Systematic Reviews. No. 6., 22. June 2018. doi:10.1002/14651858.CD012597.pub2
Saline nasal irrigation: Its role as an adjunct treatment
Disclaimer: ill people or those with recent surgery as well as extra paranoid germaphobes might want to boil the water before use, to kill any of the germs potentially present in any water, then wait until it reaches body temperature. If the municipal water supply is reasonably clean this is not necessary for healthy adults. People in third world countries like the USA may want boil the water for all the reasons above anyway.
Main sources for bacterial contamination are preparing a too large portion of solution and letting it sit and the pots used themselves:
Although common and frequently based on potentially dangerous bacteria, contamination is considered a false problem by some experts. They think that the nasal cavity is naturally full of bacteria and the addition of new pathogens is not clinically relevant.
The problem of sterility of the solutions and devices has been debated. Solutions are at risk of contamination when large volumes of solution based on distilled water, bottled water, or boiled water are prepared at home, maintained in containers and used each time when NI is needed by withdrawing the required amount of liquid. Devices can be contaminated when they are continuously used without adequate cleaning. Lee et al. reported that after one and two weeks of use, irrigation bottles used by adults undergoing endoscopic sinus surgery that were washed with hot soapy water after each use were found to be contaminated by a large spectrum of bacteria, including Pseudomonas aeruginosa, Serratia marcescens, Proteus mirabilis, and Staphylococcus aureus. Similar findings were reported by other authors and, because in many cases contaminating bacteria were the same as those that could cause acute rhinosinusitis, it was suggested that the main source of device colonization was the sinonasal cavities. The risk of contamination seems independent of the type of device. Additionally, the use of a one-way valve irrigation bottle, theoretically capable of reducing the risk of reflux of contaminated solution in the device, was found to be practically ineffective. In contrast, contamination seems to be influenced by the composition of the solution. It was shown that acidic, isotonic saline solutions were more frequently associated with bacterial contamination probably because some of the most common contaminants grow optimally in similar environmental conditions. Finally, contamination was found more frequently with longer durations of NI use. With some exceptions, studies have reported that both bottles and bulb syringes were contaminated after one to two weeks of use in approximately 25% of the cases and in 45% after four weeks.
–– Nicola Principi & Susanna Esposito: "Nasal Irrigation: An Imprecisely Defined Medical Procedure", Int J Environ Res Public Health. 2017 May; 14(5): 516. doi: 10.3390/ijerph14050516 PMCID: PMC5451967 PMID: 28492494