This letter is to inform you of the results of a series of acute inhalation studies conducted by Haskell Laboratory with anhydrous hydrogen fluoride (MF) which suggest that the toxicity of HF may be related to relative humidity. These preliminary data indicate that toxicity increases with increasing relative humidity. We believe that the test results do not support a finding of substantial risk because the new data do not constitute a significant toxicological difference from published data. We are providing these test results for your information. The data from these studies was to be used to assess the validity of a lethality value for HF provided in a United Kingdom test rule, known as Naval Engineering Standard 713, for determining the toxicity of combustion products. To assess the toxicity of HF in an environment where relative humidity can vary greatly, such as in a fire, groups of four male Crl:DE(trademark)BR rats (approx. 200-300 g) were exposed head-only to anhydrous HF for time periods of 5, 15, 30 or 60 minutes under dry (relative humidity 0.5-10%) and moist (relative humidity 40-50%) conditions. Gas samples were obtained from the exposure chamber using midget impingers; HF concentrations were measured with a fluoride ion-selective electrode. Lethality was the primary endpoint for these studies. Mortality data were obtained from 6-15 exposures per time point and LC50 values were calculated by probit analysis. Results of these studies for dry and humid air are summarized in Table I. A preliminary report of these studies is being prepared and will be forwarded to you upon completion.