California and more than two dozen other states require oil and gas producers to disclose the chemicals they use during hydraulic fracturing activities, enabling scientific and public scrutiny of the environmental and human health hazards these substances may pose. But all existing disclosure regulations cover chemical use only in hydraulic fracturing, known as fracking, and, in California, two other types of well-stimulation treatments. Many of the same chemicals used in hydraulic fracturing go undisclosed when they are used in numerous routine, unregulated oil- and gas-field activities such as the drilling, cleaning and maintenance of wells, according to a study published in PLOS ONE today (April 19).