Introduction: OSHA’s Heat Illness and Illness Prevention in Outdoor and Indoor Settings Proposed Rule requires a heat injury and prevention plan (HIIPP) for each worksite if an initial or high heat trigger are met. Initial or high heat triggers are met when the heat index or the WBGT equivalent exceeds 80°F or 90°F, respectively. Additionally, employers should establish acclimatization schedules for new workers and workers who may be gone from the worksite for extended periods. To help anticipate the exceedances of the heat triggers, we conducted a retrospective analysis to determine how many counties in each US state exceeded either of the heat triggers in the month of July from 2020-2025. In this period, there were 17,511 instances where a county in the U.S. was above the initial heat trigger based on temperature alone. Additionally, there were 6,899 instances where a county in the U.S. was above the high heat trigger based on temperature alone. Thus, a HIPP would have to be enacted for a time period for jurisdicted worksites. To better understand impacts at the local level, a case study was performed for Randolph County, West Virginia to derive the number of heat trigger days based on heat index calculations and identify instances where reacclimatization would have been needed for workers in July 2023. This retrospective analysis provides an indication of the frequency of days that the elements of a HIIPP will need to be in place to protect workers from potential heat stress hazards. Situation / Problem: OSHA’s Heat Illness and Illness Prevention in Outdoor and Indoor Settings Proposed Rule requires implementation of a heat injury and prevention plan (HIIPP) for each worksite if an initial or high heat trigger are met. Employers should review and establish a viable and effective HIIPP prior to an initial heat trigger exceedance. Each worksite is required to have a HIIPP that includes identifying either heat index or WBGT for monitoring, designating and training a heat safety coordinator, and additional requirements depending on the heat trigger. The elements of the HIIPP include access to 1 quart of drinking water per employee per hour, readily accessible break area with shade or air conditioning, acclimatization schedules for workers, rest breaks, communication plan and PPE maintenance. Additional elements for the high heat trigger include minimum frequency and duration for rest breaks, monitoring of employees for signs/symptoms of heat related illness, and posting of warning signs in indoor work areas. Effective implementation of the HIIPP requires planning and mobilization of many resources. Therefore, anticipating the potential frequency of heat trigger exceedances is important for resource planning and ensuring that the HIIPP can be effectively implemented. As such, a retrospective analysis of heat trigger exceedances in the U.S. was conducted to evaluate its usefulness in helping industrial industrial hygienists anticipate the need for implementation of a HIIPP. Methods: We performed a search of publicly available meteorological databases related to heat indices, heat stress, and related metrics. We then assessed data availability for outdoor wet-bulb globe temperature (WBGT) and related metrics. Based on data availability, we searched for and identified ambient temperature and relative humidity databases to calculate heat indices. For the initial analysis, we determined how many counties from July of 2020-2025 in each US state where an initial and high heat trigger instance (monthly maximum temperature exceeded the designated heat index temperature) occurred based on ambient temperature and would have required a HIIPP for outdoor worksites. We used the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental information, Climate at a Glance: County Mapping database to derive the maximum ambient temperature in July from 2020-2025 for every U.S. county. This allowed us to identify if there was an instance when the HIIPP plan would be in place for either the initial or high heat trigger. We then calculated the total number of counties that exceeded either trigger in every state based on maximum ambient temperature. To understand impacts at a local level, we conducted a case study wherein we identified a county within our 2020-2025 dataset that had a maximum ambient temperature below the initial heat trigger, but with historically warm, humid summers. Randolph County, West Virginia was identified, and the maximum ambient temperature and corresponding year was selected. Ambient temperature was derived from the NOAA National Centers for Environmental information, Climate at a Glance: County Mapping database and relative humidity was derived NOAA National Centers for Environmental information, U.S. Local Climatological Data (LCD) database. The average monthly heat index for July 2023 was calculated for this county using Lu and Romps’ heatindex R package (2025). Additionally, daily heat indices were calculated. We then identified potential instances where according to the OSHA proposed rule, workers would have needed to be reacclimatized in July 2023. Results / Conclusions: From 2020-2025, the majority of US states met the initial heat trigger in the summertime based on temperature alone. In July of 2020-2025, there were 17,511 instances where a county in the U.S. was above the initial heat trigger based on temperature alone. Additionally, there were 6,899 instances where a county in the U.S. was above the high heat trigger based on temperature alone. Fifteen states had at least one year where all counties exceeded the initial heat trigger in July. The state of Texas had the most exceedances, with all 254 counties exceeding the initial heat trigger for the month of July from 2020-2025 and all 254 counties also exceeding the high heat trigger in July of 2022 and 2023. In our case study of Randolph County WV, the July average monthly heat index was 83.8°F, which exceeded the initial heat trigger, indicating many potential days where implementation of a HIIPP would have been necessary for outdoor workers. Challenges with conducting the retrospective analysis were primarily related to availability of information. Specifically, we did not identify any publicly available database that compiles historic heat indices. Many heat stress and heat index resources and apps are based on real time monitoring but do not have a function to retrieve historic data. Additionally, we found that data availability varied by county and state. When identifying a county for the case study, there were several instances where either ambient temperature or relative humidity data was not easily and publicly available depending on the county and time period. Additionally, the ambient temperature and relative humidity datasets use in this analysis were not collected synchronously, and the exact locations of where the ambient temperatures were measured was not available. As such there is a potential for data misclassification at the site-specific level. In this analysis, we analyzed available databases to understand heat trigger exceedances in July from 2020-2025. This exercise identified the high frequency of exceedances across the U.S and provides a screening level method for anticipating the frequency at which HIIPP implementation may be needed and allow for proper resource planning to protect workers from heat related illnesses. Additionally, we identified the need for a focused, publicly available database or an additional feature to existing heat stress resources where historic data related to heat index is readily available. We are prepared to discuss the result in context with the HIIPP requirements, and how having a robust HIIPP is imperative to protect worker health.