Used for cleaning masonry, muriatic acid must be handled with care. A dilute form of hydrochloric acid, it can liberate both hydrogen and chlorine gases. Hydrogen is flammable and chlorine is highly toxic. Before using it, dilute further with water, adding the acid to a measured amount of water, never the other way around. Take steps to protect nearby metalwork and plants. When you're finished, dispose of the unused acid by carefully neutralizing it with gardening lime or taking it to a hazardous waste handler.
Highly concentrated muriatic acid can burn skin and other body tissues. Always wear protective clothing, gloves and eye wear when handling it.
When diluting muriatic acid with water, don't add water to acid. The mixture may erupt, spraying you with acid. Instead, always add acid to water.
Muriatic acid can corrode and weaken metals. When cleaning with it, protect any nearby structural metalwork with plastic.
The mist and vapors from muriatic acid are also hazardous. If you use it, make sure there's plenty of ventilation. Avoid using it on days when the wind can carry it.
If you find an old, half-filled bottle of muriatic acid, don't open it. The chlorine in the acid will, over time, evaporate from the liquid into the empty space. Chlorine gas is reactive and toxic. Take the bottle to a hazardous waste handler.