Understanding the biological effects of acute ultraviolet-B (UV-B) exposure requires understanding the typical intensity and duration of such exposures. The occurrence of high hourly biologically effective UV-B (UV-Bbe) exposures was evaluated using two response functions (1971 and 2003) for the 1997–2002 summer growing seasons (May–August) at five locations across the continental United States. The frequency of occurrence of the upper 5% of all seasonal UV-Bbe hourly exposures of 1 h to 4 h duration in 1 day and repeating the same exposure over consecutive days was evaluated. High hourly UV-Bbe exposures occurred most frequently during June and July. There was a 30% frequency of occurrence of a day during the growing season with 2 h of hourly exposure in the upper 5% of UV-Bbe (1971) values across any of the five locations studied. The frequency of occurrence of 2 h of UV-Bbe (2003) exposure in the uppermost 5% of all observed hourly values was 14%. An approach and specific experimental square-wave enhancement exposure regimes that are consistent with the range of actual exposures and total ozone column (TOC) during the May through August period are provided. A 2 day high UV-B event with 2 h of high UV-Bbe occurred at least 10% of all days in the growing season, representing a reasonable short-term high-exposure regime. Different exposure statistics and resulting enhancement regimes would likely result if only June and July were included in the analysis.
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