Q After root canal work, the dentist cleaned it out with hydrogen peroxide, then sodium hypochlorite and saline. But is sodium hypochlorite safe? I asked him not to use it on me again. - BJ, India

A Dentists still regularly use sodium hypochlorite for canal irrigation because it is an effective antimicrobial agent. It is the irrigating solution of choice in dentistry, and the conventional view is that it is safe, and no more an irritant than the usual saline solution, provided it is handled with care.

There are dentists who use it with hydrogen peroxide which, they believe, neutralises any dangers. But this might be just wishful thinking. Sodium hypochlorite is the chemical name for household bleach (NaOCl - one atom each of sodium, oxygen and chlorine).

Not surprisingly, some dentists regard it as toxic and dangerous. Common reactions to it include gastrointestinal irritation with nausea, vomiting and diarrhoea, but usually only if the solution is swallowed. A high-concentration solution of sodium hypochlorite can result in corrosive burns to the skin or gums, and eye damage.

However, inhalation of sodium hypochlorite can cause burning in the throat and coughing due to the chlorine gas. High exposure can lead to swelling and obstruction of the airways. Congestive heart failure and pulmonary oedema are rare reactions, but can occur from just inhaling the solution.

Dentists should immediately check a patient’s lung function and carry out a chest X-ray if there is even a moderate reaction to the solution.

Extraordinarily, some dentists inject the solution into the gums. One website warns dentists: 'There generally is an immediate severe burning sensation often accompanied by very rapid gross swelling.'