Heat exposure impairs decision making and increases injury and death risk among outdoor workers. Since 2022, five US states have adopted outdoor heat standards, but these recent policies cannot yet be evaluated with the available data. We examined California's 2005 outdoor heat standard, which is the only standard that was in place before 2022. Using negative binomial regression with wild cluster bootstrapped standard errors, we compared heat-related deaths among outdoor workers in California with those in neighboring states during the period 1999-20. California's policy showed increasingly strong associations with reduced deaths over time, with no decline during initial implementation (2005-09) but estimated reductions of 33 percent after enforcement increased (2010-14) and 51 percent after policy revisions (2015-20). Although these period-specific reductions were not individually statistically significant, a Wald test indicated that the combined effect during the period 2010-20 was statistically significant. These findings suggest that when properly designed and enforced, comprehensive heat standards can protect vulnerable workers as temperatures rise.