The potential for exposure to toxic chemicals exists during the exploration, production, and distribution phases of petroleum refining. "Potential pulmonary toxicants" include irritant gases, sulfur compounds, acids, carcinogens, particles, fibers, and sensitizers. "Significant reductions in exposure have resulted in the past 30 years, with better control of fugitive emissions, minimization of benzene exposure, and automation of refinery operations." [Harber, p. 688-96] Refinery products include gasoline, other fuels, lubricants, waxes, asphalt, coke, and chemical intermediates. Crude oil and natural gas contain about 0.1% to 3% benzene. Refinery operations include distillation, cracking/conversion, treating, and blending. [PMID 20941467] Oil and natural gas reserves increased 63% and 17%, respectively, in the U.S. between 2009 to 2010 secondary "to new accessibility to tight shale formations using horizontal drilling and high volume hydraulic fracturing." [PMID 24634090] "Approximately 562,000 workers were employed in the U.S. oil and gas extraction industry in 2012; nearly half of those workers were employed by well servicing companies, which include companies that conduct hydraulic fracturing and flowback operations." [PMID 25175286] See "Crude oil."