Japan’s new reporting requirements for mercury will affect nearly half of all business entities that keep or use the toxic metal, regulators say.

Companies whose stored mercury goes above a new 30-kilogram (66-pound) threshold are required to report on their mercury management by June 30 under the Mercury Pollution Prevention Act, which went into effect Aug. 16, 2017.

Mercury is a neurotoxin that at sufficient exposure levels can affect coordination, cognition, memory, and vision in addition to...