Steam crackers convert hydrocarbon feedstock (e.g., natural gas liquids) to light olefins via thermal cracking and produce hydrogen as a by-product during the process. Benefiting from the shale gas boom in recent years, the overall production capacity of U.S. steam crackers, as well as the potential of by-product hydrogen production, is continuously growing. We estimate that 3.5 million tonne/year of by-product hydrogen can be produced from steam crackers, almost doubling the size of the existing U.S. merchant hydrogen market. We also find that producing hydrogen from steam crackers creates less (15%-91%) life-cycle greenhouse gas emissions than the conventional centralized steam methane reforming (SMR) pathway. For criteria air pollutants, life-cycle emissions reduction benefits vary greatly (-75% - +85%), depending on the co-product treatment scenario (substitution or allocation) and air pollutant type. The substitution scenario generally results in an increase of criteria air pollutants emissions, mainly due to the requirement of substitutive natural gas fuel. We estimate that the cost of purified by-product hydrogen fuel from steam crackers is $0.9-1.1/kg, reducing hydrogen production costs by 30% compared to the conventional central SMR pathway. Furthermore, using by-product hydrogen from steam crackers can generate credits of $1.8-2.5/kg under California's low-carbon fuel standard. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.