Dry Eye Dx and Tx
Humidity or the Lack Thereof
By William Townsend, OD, FAAO
Patients often ask why their dry eye symptoms worsen during the winter months. To answer that question, we need to understand the environmental factors that promote ocular surface dryness.
Surface water evaporates and enters Earth's atmosphere as water vapor. Absolute humidity, expressed as grams of water vapor per cubic meter of air, depends primarily on altitude, but also temperature and water vapor concentration. Absolute humidity at 5,000 feet is less than at sea level simply because air pressure decreases as altitude increases.
Relative humidity measures water vapor as a percentage of saturated conditions, i.e., 100 percent relative humidity. It depends on air temperature but is not affected by altitude. In a given location, the relative humidity in summer can be the same as in winter, but the difference in the amount of water vapor present is profound. The crucial difference is temperature. At 80 degrees Fahrenheit, saturated air contains 22 times more water compared to saturated air at 0 degrees Fahrenheit.
To appreciate the impact of heating and cooling systems on ocular dryness, we need to understand how humidity and temperature affect comfort. The primary mechanism by which the body regulates temperature is evaporative cooling. Humans perceive the effect of heat transfer from the skin rather than temperature per se, so we feel warmer when the humidity is high. For example, when the room temperature is 70 degrees and the relative humidity is 20 percent, it feels like 66 degrees. When the relative humidity increases to 80 percent, however, the temperature in the same room feels like 71 degrees.
Air conditioning lowers temperature and relative humidity, creating the sensation of cooler skin, but it can also exacerbate dry skin and dry eyes. Heating systems can also deplete moisture in the air. How are these factors relevant to clinical eye care?
Impact on Dry Eye
Sunwoo et al (2006) evaluated the impact of low relative humidity on various physiological and subjective responses. They found that relative humidity less than 30 percent causes the eyes and skin to become dry. At less than 10 percent relative humidity, the nasal mucous membrane also becomes dry, and the mean skin temperature decreases. They also found a clinically significant increase in blink rate at 30 percent and 10 percent relative humidities. They concluded that a relative humidity of greater than 30 percent is necessary to avoid dry eyes and skin.
Meibomian Gland Dysfunction
Evaporation increases as humidity decreases. Arciniega et al (2011) evaluated the effect of evaporation on meibomian gland secretions in individuals who have keratitis sicca (KCS) and KCS with meibomian gland dysfunction (MGD) at several levels of relative humidity. They found increased evaporation in both groups compared with controls. All groups showed lower evaporation at higher relative humidity levels. Manual expression of the meibomian glands resulted in reduced evaporation in the controls and in the KCS with MGD group but not in the KCS group. These findings suggest that improving meibomian gland secretions can be beneficial in individuals who have MGD and in those without dry eye. This is especially true when relative humidity is low.
Minimize Humidity's Impact
Numerous environmental factors can affect ocular dryness. Understanding these issues can help us make appropriate recommendations to minimize the negative effect of low relative humidity for all of our patients, including contact lens wearers. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #195.
|Dr. Townsend practices in Canyon, Texas, and is an adjunct professor at the University of Houston College of Optometry. He is president of the Ocular Surface Society of Optometry and conducts research in ocular surface disease, lens care solutions, and medications. He is also an advisor to Alcon, B+L, CooperVision, Tearlab Corporation, and Vistakon. Contact him at firstname.lastname@example.org.|