"Evaporation" is the change in the structure of water from a liquid state into a vapor or gaseous state. All liquid and frozen water evaporates pretty much all the time. However, certain environmental factors, such as dry surrounding air or warm temperatures, will accelerate the process.
Evaporation of water from the body into the atmosphere is a constant for all living things and the lifelong objective of living organisms is to slow evaporation. When the rate of water evaporation exceeds the organism's ability to replace the water, death occurs. An organism whose water content has dropped below the optimal level required to maintain healthy functioning, is considered "dehydrated."
Bio-Logic Aqua Research has discovered that dry eye is a dehydration disease resulting from accelerated evaporation. The specific role of evaporation in dry eye is explained in the accompanying excerpts from Tear Film and the Treatment of Dry eye Disease, by William D. Mathers, MD, a continuing education course for pharmacists sponsored by RxSchool.com and Bio-Logic Aqua Research, 2004. The sections in the book on misting and evaporation were based on unpublished research conducted by Dr. Mathers at Oregon Health and Sciences University - Casey Eye Institute in 1999.
All liquid-to-gas interfaces, according to Dr. Mathers, have a vapor pressure that is temperature dependent and controls the rate of liquid-to-gas phase transition. Water, therefore, evaporates into the air until the vapor pressure equilibrates and the air is saturated. This is achieved only when the humidity reaches 100%, which rarely occurs. The surface of the eye (which is 99% water) is subject to evaporative loss that would include the total normal tear production if the surface were not protected by the lipid (oil) layer. A normal lipid layer reduces evaporation extremely effectively, by 95%, and keeps the eye wet.
Evaporation from the ocular surface, Dr. Mathers explains, can be measured by isolating the air around the eye and monitoring the rise in relative humidity. Evaporation will cause the humidity to rise and the rate of rise reflects the evaporation rate. The normal rate of evaporation from the surface of the eye is 15 x10-2 grams/cm2/second. This was determined from Dr. Mathers' work but there has been reasonable agreement between most research groups that have investigated evaporation.
A normal tear film evaporation rate is the equivalent of 0.15 microliters per minute. This amounts to 36% of the combined total of tear flow and evaporation in a normal subject, by the Dr. Mathers' estimation. The normal eye loses a relatively small amount to evaporation.
Evaporation is usually increased in dry eye, says Dr Mathers. This results in a much higher percentage of the total tear production lost to evaporation since less tear is also produced.
Evaporation is an important cause of dry eye for two reasons:
1. Some environments are very dry such as deserts, winter air heated to room temperature, and airline cabins. Liquid water is most likely to evaporate in the presence of dry air, warm temperatures and wind.
2. Evaporation rates can increase dramatically in some subjects with meibomian gland dysfunction.
The surface of the eye would quickly dry out were it not for the protective effect of the lipid layer that reduces evaporation. In the normal eye, only a small percentage of tear production evaporates but in dry eye this can increase to include nearly the total amount produced. Subjects with dry eye that have had both upper and lower punctum occluded, usually do not suffer from epiphora (tearing) because all their tears either evaporate or are absorbed by the conjunctiva.
Reflexive blink rate is an important key in avoiding dry eye symptoms, according to Bio-Logic Aqua Research. Blinking recharges the tear film and spreads the lipid layer. When blinking ceases, the ocular surface will begin developing dry spots and become uncomfortable after about ten seconds. This is called "breakup time." With dry eye, the breakup time is even shorter.
Computer operation, TV watching and many other activities involving intense concentration and staring at a bright light, can reduce the number of reflexive blinks per minute from a normal of 20 to 30 to as low as three. Blink rate reduction can profoundly affect the tear film's evaporation rate.