If possible, do not use toners that require heating. Add acid to water. Do not contaminate sulfide or selenium toners with acids. Selenium toners give off large amounts of sulfur dioxide gas. Exposure to selenium salts to acid will result in toxic hydrogen selenide gas being produced.
Sulfides release highly toxic hydrogen sulfide gas during toning or when treated with acid.
For example, with two bath sulfide toners, make sure that you rinse the print well after bleaching in acid solution before dipping it in the sulfide developer. Avoid thiourea whenever possible because of its probable cancer status. Ensure good ventilation of the darkroom. Wear gloves and goggles. Cover all solutions when not in use to prevent evaporation or release of toxic vapors and gases.