International trade in sulfuric acid is regulated by the 1988 United Nations Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances, which lists sulfuric acid in Schedule II of the Convention as a chemical frequently used in the illicit manufacture of narcotic drugs or psychotropic substances. [
Also, since acid is denser than water, it will sink to the bottom. Heat is generated at the acid and water interface at the bottom of the vessel. Acid doesn't boil because it has a higher boiling point. Warm water near the interface rises due to convection, which cools the interface and prevents the acid or water from boiling.

Conversely, adding water to concentrated sulfuric acid creates a thin layer of water on top of the acid. The heat generated in this thin layer of water would boil, causing sulfuric acid aerosols to spread, or worse, cause an explosion.

Preparation of solutions with a concentration greater than 6 M (35%) is the most dangerous, as the heat generated may be sufficient to boil the diluted acid: effective mechanical stirring and external cooling (such as an ice bath) are essential.

The rate of the reaction doubles for every 10°C or so increase in temperature. [41] Therefore, unless the mixture is given time to cool, the reaction will become more vigorous as the dilution proceeds. Adding acid to warm water can cause a violent reaction.

On a laboratory scale, sulfuric acid can be diluted by pouring the concentrated acid over crushed ice made from deionized water. The ice melts in an endothermic process while dissolving the acid. The heat required to melt the ice is greater than the heat given off by dissolving the acid in the process, so the solution remains cold. After all the ice has melted, it can be further diluted with water.
The main occupational risks posed by this acid are skin contact (see above) leading to burns and inhalation of aerosols. Exposure to high concentrations of aerosol causes immediate and severe irritation of the eyes, respiratory tract and mucous membranes: this irritation ceases rapidly after exposure, but if tissue damage is more severe, there is a risk of subsequent pulmonary edema.