Change in Aqueous Tear Evaporation with Change in Relative Humidity

Abstract

Abstract: : Purpose: To verify the relationship between relative humidity and aqueous tear (AT) evaporation. Methods:The data was taken from our evaporometer database, compiled from several different clinical studies, spanning a variety of dry eye diseases and severity of symptoms as well as a wide age range (47.6 ± 20.4 years). The data pool consisted of 128 females and 201 males; 156 measurements were from dry eye patients and 173 from normal patients. Factors such as room temperature and light remained constant over the experimental span of 18 months. A humidity sensor was mounted 1.5cm from the ocular surface inside an eye goggle designed to provide a seal by conforming to the contours of the face. With the patient’s eye closed, desiccated air was pumped into the goggle until the relative humidity (RH) dropped below 15%. The air flow was stopped, and, with the patient’s eye still closed, the RH was recorded versus time until the RH reached 45%. This process was repeated with the patient’s eye open during the recording phase. The patient’s exposed ocular surface area was measured by digital photography, allowing for an accurate subtraction of the contribution to evaporation from the exposed skin within the eye goggle. Evaporation was calculated from the rate of change of RH in two different ranges, 25% to 35% RH and 35% to 45% RH, and reported in µl of water evaporated per cm² of exposed ocular surface per minute. Results: The rate of evaporation in the 25% to 35% RH range was 0.047±0.025 µl/cm²/min. The evaporation in the range of 35% to 45% RH was 0.033±0.017 µl/cm²/min. Thus a small reduction of relative humidity (10%) from 35–45% to 25–35% resulted in an average increase in evaporation of 40.8%. Although these two humidity ranges were contiguous in a single measurement, the difference in the corresponding rates of evaporation was statistically significant (P<0.001). Conclusions: We have experimentally demonstrated that a decrease in humidity leads to a significant increase in aqueous tear evaporation. This inverse relationship of AT evaporation and humidity appears to become more pronounced as humidity is lowered but plateaus as humidity is increased. The drastic jump in evaporation by 40% with a decrease from 35–45% to 25–35% in relative humidity suggests that humidity may play a greater role in the severity of dry eye symptoms than previously thought.

Keywords: aqueous • stress response