Low relative humidity conditions, such as those experienced in airplane cabins during flight, cause an increase in aqueous tear evaporation that intensifies dry eye symptoms, according to a study.
Eduardo Uchiyama, MD, and colleagues at the University of Texas Southwestern Medical Center evaluated 29 subjects in the prospective, experimental laboratory study. Eighteen of the subjects had a history of dry eye syndrome, and 11 were considered healthy. Both eyes in each patient had similar levels of dry eye symptoms.
The study was published in Eye & Contact Lens.
The tear film is exposed to great disparities in relative humidity in different environmental conditions, and when exposed to air with less than 100% relative humidity, it is subject to evaporation. One particular area in which many will suffer some form of eye dryness and discomfort is on a commercial flight.
“Whenever the humidity or environment is lower than 100%, any aqueous surface is going to evaporate,” Dr. Uchiyama said in a telephone interview with Ocular Surgery News. “The lower the relative humidity, the higher the rate of evaporation. Let’s say you have a rate of evaporation of 20%. If we compare that against the rate in an arid area with a relative humidity that could be around 15%, the evaporation is going to be much higher. … If you decrease the relative humidity, the amount of tears you’re going to lose through evaporation is going to be much higher.”
Airplane cabin simulation
Based on the results of a clinical examination, patients with dry eye were assigned either to a group made up of subjects with classic keratoconjunctivitis sicca for normal appearing meibomian secretions or to one with keratoconjunctivitis sicca with meibomian gland dysfunction for meibomian glands that were difficult to express or were turbid on expression.
The 11 healthy subjects with no evidence of ocular surface or drying disorders were part of the control group.
An evaporometer (Oxdata) used an air pump to direct air through a drying tube into an eye goggle containing a sensor for humidity and temperature. The dry air was pumped into the goggle until the relative humidity was lowered to 15%.
The researchers then monitored increases in humidity from skin and ocular surface evaporation during simulations of relative humidity in an airplane cabin, estimated to be between 20% and 25%, or an arid region, estimated to be between 40% and 45%. They then digitally stored the data.
The first experiment was performed with the eyelids closed and then opened, Dr. Uchiyama said. The tear component of evaporation was analyzed by calculating the difference between the two results, measuring the ocular surface using digital photography and estimating tear film evaporation under two ranges of increasing relative humidity.
Increases in evaporation were measured in each group and compared statistically against each other. The authors said average humidity levels in aircraft cabins during commercial flights generally range from 9% to 28% at average temperatures of 23° C to 24° C.
“If you have an airplane that is completely empty and it’s flying, the humidity can be as low as 1%. We thought those numbers were too low. The closest we could approximate was a range between 20% and 25%. We found 100% more evaporation by lowering the humidity to that point,” Dr. Uchiyama said.
Dry eye symptoms that result from this evaporation “have a direct correlation with the duration of the flight and occur in many people who otherwise do not have dry eye complaints or problems,” he said.
Environment vs. dry eye
According to Dr. Uchiyama, the main objectives of the study were to determine how important environmental conditions such as low humidity, wind and close working environments were in relation to dry eye syndrome and to find new ways of decreasing evaporation in dry eye patients.
“There are many people that we see in our clinic that usually don’t have any dry eye symptoms,” he said. “Whenever they fly or are in an environment where the humidity drops, they start feeling some burning sensation and drying in their eyes.”
The researchers determined that evaporative rates under the two humidity conditions were “statistically significant” and similar in all groups when eyes were exposed to lower relative humidity. In the control group, the rate increased by 85.59%, while the classic keratoconjunctivitis sicca group rose by 117.08% and the keratoconjunctivitis sicca with meibomian gland dysfunction group increased by 97.92%. Mean increase across all study subjects was 99.72% (P = .001), the authors said.
Based on these findings, they concluded, “These studies quantitatively show the negative impact of environmental low relative humidity conditions, including those associated with commercial airplane travel, on aqueous tear evaporation dynamics. The increased evaporative rate is similar in healthy subjects and patients with dry eye. These findings provide useful data for the development and evaluation of treatment paradigms for any person who notes dry eye symptoms in low relative humidity environments.”
Although other factors contributing to increased evaporation rates are likely to be present in non-laboratory, low relative humidity conditions, the authors said they believed that low relative humidity alone is enough to cause large increases in evaporative rates in subjects with and without dry eye. They also said increased evaporation is important in the pathogenesis of dry eye syndromes and that it could be an exacerbating factor as well in healthy subjects.
“The relative humidity impact was as significant for healthy subjects as that seen in patients with dry eye,” they reported. “Although the difference between the two relative humidity experimental conditions was only 20%, this change in relative humidity had a statistically significant impact on the evaporative rate for each of the three study groups.”
Although no anti-evaporative effect has been shown with available medications, the authors said artificial tears should decrease dry eye symptoms by adding moisture to the ocular surface, at least temporarily, and that moisturizing eye drops should be considered for low relative humidity conditions.
“Sometimes they need to be blinking more. A dry patient will blink and feel it,” he said. “I have patients who use drops every hour. Even with drops, they can’t have a normal life.”
Dr. Uchiyama and colleagues plan to investigate ways of preventing or decreasing the evaporation rate in healthy and dry eye patients.
“It would be good to have a way of avoiding these symptoms, especially if you have dry eye and have a low threshold,” he said. “You know that whenever you go onto a plane, you are going to feel that, and it is something you don’t want to experience.”
For more information:
- Eduardo Uchiyama, MD, can be reached at the University of Texas Southwestern Medical Center, 5323 Harry Himes Blvd., Dallas, TX 75390 U.S.A.; +1-214-648-3407; fax: +1-214-645-9552; e-mail: firstname.lastname@example.org.
- Uchiyama E, Aronowicz JD, Butovich IA, McCulley JP. Increased evaporative rates in laboratory testing conditions simulating airplane cabin relative humidity: An important factor for dry eye syndrome. Eye Contact Lens. 2007;33:174-176.
- John Misiano is an OSN Staff Writer who covers all areas of ophthalmology.