Background and Purpose: Many state air toxics regulatory programs require inhalation reference criteria protective of acute (24-hour) exposures, which are challenging to determine for lead (Pb), as the most sensitive endpoint is neurotoxicity associated with increased blood Pb in children. One approach is to use the National Ambient Air Quality Standard (NAAQS) for Pb of 0.15 μg/m3, which was used by the Oregon Department of Environmental Quality. However, the NAAQS is intended for a 3-month average concentration, which is not well suited to characterize 24-hour exposures. An alternative approach to setting an acute threshold for Pb was developed, using the EPA’s All Ages Lead Model (AALM) Version 3.1, which allows users to simulate single-day exposures (EPA, 2025). Biokinetic modeling was performed to predict the air concentration for one day of exposure that would result in a blood Pb increase of 1 μg/dL. The California Office of Environmental Health Hazard Assessment (OEHHA) uses 1 μg/dL as the threshold for a source-specific incremental increase in blood lead levels (BLLs) for protection of school children and fetuses (OEHHA, 2007). Methods: EPA’s AALM was used to estimate BLLs for children each year for birth to 6 years because children are more sensitive than adults, and kinetic and exposure parameters change as children grow. First, baseline BLLs were modeled considering all exposure routes in the AALM (e.g., soil, indoor dust, food) except for air, using default model inputs (EPA, 2024). The upper tolerance limit for background Pb in Oregon soil of 79 μg/g was used for modeling soil and dust exposures. The AALM was subsequently run in an iterative process by changing the air concentration on a single day until the estimated BLLs corresponded to a BLL of 1 μg/dL above baseline. The single day of exposure (i.e., acute pulse) was simulated separately for each year from year 0 to year 6, such that there were seven different model simulations. To simulate an acute, 24-hour exposure scenario for one age group, the air concentration was assumed to be 0 μg/m3 except for the single day of exposure for the age group of interest. As shown by the magnitude of the results of the pulsed modeling, a default air concentration for Pb (0.01 μg/m3) would not contribute significantly to BLLs. The AALM uses masks to set exposures for specific repeating periods. In this case, an annual mask was set to 0 μg/m3 for days 2 through 365 of each year. Day 1 exposure was set to 0 μg/m3 for all age groups except for the specific age group with the acute 24-hour exposure. The year with acute/pulsed exposure had a non-zero air concentration on the first day. For example, for year 1, the air concentrations on day 1 was modeled for three age groups: (1) age 0 at 0 μg/m3, (2) age 1 at the non-zero air concentration, and (3) age 2 and beyond at 0 μg/m3 through the end of the simulation (6.1 years). Default lung parameters from AALM 3.0 were selected (EPA 2024, 2025). These parameters represent lung kinetics for ultra-fine (approximately 0.1 μm in diameter) combustion aerosols, and relative bioavailability for inhalation exposure was assumed to be 1 (EPA, 2024). Results: Baseline BLLs ranged from 1.10-2.21 μg/ dL. The day 1 air concentration that resulted in a predicted BLL closest to the target increased BLL ranged from 3.86 μg/m3 (year 0) to 7.64 μg/m3 (year 5). These values are all well above the NAAQS for Pb of 0.15 μg/m3. AALM predicted that the highest BLL following acute air exposures occurs two days after the 24-hour pulse exposure. The single-day air concentration for the most sensitive age group (newborn, 0-year age group) associated with a 1 μg/dL increase from the baseline BLL was 3.9 μg/ m3 for the newborn age group. The estimated BLLs decreased to baseline BLLs within approximately 20 days, and the peak BLL was 3.21 μg/dL, which is lower than the CDC Pb reference level of 3.5 μg/dL. Conclusions: An acute 24-hour threshold for Pb in ambient air of 3.9 μg/m3 would be appropriate and protective for all ages, specifically children. This guideline is complementary to the NAAQS because a single pulsed 24-hour concentration of 3.9 μg/m3 would not exceed the NAAQS when averaged with background Pb in air concentrations over three months.