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A. Tomlinson, L. C. McCann, P. A. Simmons; Effect of Humidity on Human Tear Film Evaporation Within a Controlled Environment Chamber. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4143.
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Many dry eye patients are sensitive to adverse environments such as low humidity and wind, in which tear film evaporation (TFE) increases. Quantitative data is lacking on how the TFE responds to environmental change. We have used a controlled environment chamber to study these effects, including the time of adaptation of TFE to environmental conditions, and the difference between normal and dry eye subjects in their TFE responses.
Two studies were carried out. Initially, the TFE adaptation time was determined by exposing 5 normal subjects to a humidity of 40% at a temperature of 72°F in the chamber for periods of 0, 5, 10, 15, 20 and 25 minutes, with TFE measurement at each time point. Then a second experiment varied the humidity from 5% to 70% (at 72°F), and measured the TFE response in 20 subjects (10 normals and 10 dry eye (DE)) after the required adaptation time.
In the initial study of adaptation, evaporation at 40% humidity showed a peak (around 5 minutes), followed by a decline to the steady state level at 10 minutes. The effect of different humidity levels in the chamber on TFE was as expected, with higher levels at low humidity. At 70% humidity the evaporation was effectively zero. The difference between normals and DE was in the direction of greater evaporation in DE. This was the case at 40% humidity (and 5%) but not at 70%, where TFE declined to zero in both groups.
The initial increase and peak in TFE during adaptation may be due to initial contributions from the moisture of the skin in the goggle, which was reduced with acclimatisation to the chamber humidity. For this environmental chamber and TFE measurement system, at least 10 minutes is required to obtain a steady-state TFE. As expected, TFE has a reverse correlation with environmental humidity in the range of 5-70%, with tear film evaporation is effectively reduced to zero at 70% humidity. As expected (because of the evaporative DE component to most patients dry eye aetiologies), DE subjects had a higher TFE at humidity levels below 70%.
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