PERCUTANEOUS-ABSORPTION OF 2,4-DICHLOROPHENOXYACETIC ACID FROM SOIL WITH RESPECT TO SOIL LOAD AND SKIN CONTACT TIME - IN-VIVO ABSORPTION INRHESUS-MONKEY AND IN-VITRO ABSORPTION IN HUMAN SKIN

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), used for control of weeds in agriculture, forestry, and rights of way, can accumulate as a residual chemical in soil. The objective was to determine percuta neous absorption of 2,4-D from soil, with emphasis on soil load and sk in contact time. With control acetone vehicle, in vivo absorption of 2 ,4-D in the rhesus monkey was 8.6 +/- 2.1% of the dose, which compared closely to published human absorption of 6.0 +/- 2.4%. Percutaneous a bsorption from soil loads of 1 and 40 mg/cm(2) were 9.8 +/- 4.0 and 15 .9 +/- 4.7%, respectively, values similar to acetone vehicle. In vitro absorption in human skin calculated from skin content accumulation ov er 24 h was 1.8 +/- 1.7, 1.7 +/- 1.3, and 1.4 +/- 1.2% for soil loads of 5, 10, and 40 mg/cm(2), respectively. Thus, soil load did not affec t 24-h, percutaneous absorption. Current Environmental Protection Agen cy (EPA) recommended calculated reductions doe to soil. load are not S upported by these results with 2,4-D. Percutaneous absorption of 2,4-D from acetone vehicle for 8 h dosing period was 3.2 +/- 1.0%, one-thir d the value of 8.6 +/- 2.1% over 24 h. With soil vehicle, absorption f or 8 h was only 0.03 +/- 0.02% for 40 mg/cm(2) soil load and 0.05 +/- 0.004% for 1 mg/cm(2) soil load. Absorption for 16 h was 2.2 +/- 1.2%. Absorption over time was linear for acetone vehicle, where total dose is deposited on skin, but not linear for soil vehicle, which had an 8 -h delay (lag time). This equates with a normal 8-h work day where mos t of the contaminated soil can be washed off the skin. The apparent pa rtition coefficient of 2,4-D between soil and water changed over time. This suggests there is a ''mobility'' phase for 2,4-D in soil that wi ll change with time. For soil vehicle, percutaneous absorption of 2,4- D was not linear in respect to soil load or to skin contact time. Calc ulation based on assumed linearity can falsely estimate potential huma n health hazard. Clearly, the dermatokinetics with soil and skin repre sent complex interactive forces that require detailed evaluation befor e overgeneralizing rules for interpretation in terms of risk assessmen t.