title: Factors affecting evaporation rates of tear film components measured in vitro.
abstract: OBJECTIVE
  With increasing age and in patients affected with dry-eye symptoms, the human tear film becomes more unstable and exhibits shorter tear break-up times. We examined whether the inclusion of proteins and lipids to water affected the evaporation rates measured in vitro and could account for the lower rates reported previously from in vivo measurements. The impact of temperature, air flow, and humidity on the evaporation rate of tears was measured in vitro.
  
  
  METHODS
  Lipid-protein interactions were measured using fluorescence spectroscopy and in vitro rates of evaporation were performed gravimetrically.
  
  
  RESULTS
  Human reflex tears evaporated at a rate similar to that of water. A temperature increase from 25 degrees C to 34 degrees C caused a threefold increase in the evaporation rates of tears in still air. Further increases were observed under a current of dry air. Wax, mucin, lysozyme, or beta-lactoglobulin did not influence significantly the rates of evaporation measured in vitro. In contrast, lipid layered on the surface resulted in a 23% decrease in the rates.
  
  
  CONCLUSIONS
  Environmental factors affect evaporation rates significantly and should be carefully controlled when performing in vivo measurements. The presence of a lipid layer lowers evaporation rates. The significantly lower rates of evaporation of tears measured in vivo suggest that with the lipid layer intact, the high reserve capacity of the lacrimal gland to provide both unstimulated and stimulated tear flow is more than enough to compensate for evaporative loss. However, with dry eye, increased rates of evaporation and decreased lacrimal tear flow could result in decreased break-up times.
authors.affiliation: Department of Ophthalmology, University of Louisville, Louisville, KY, USA. borchman@louisville.edu
authors.forename: Douglas
authors.initials: D
authors.lastname: Borchman
authors.forename: Gary N
authors.initials: GN
authors.lastname: Foulks
authors.forename: Marta C
authors.initials: MC
authors.lastname: Yappert
authors.forename: Jonathan
authors.initials: J
authors.lastname: Mathews
authors.forename: Kim
authors.initials: K
authors.lastname: Leake
authors.forename: Jim
authors.initials: J
authors.lastname: Bell
mesh_terms: D000388:Air; D005136:Eye Proteins / Q000494:pharmacology; D006801:Humans; D006813:Humidity; D006838:Hydrocarbons / Q000494:pharmacology; D066298:In Vitro Techniques; D007782:Lactoglobulins / Q000494:pharmacology; D008055:Lipids / Q000494:pharmacology; D008297:Male; D008875:Middle Aged; D009077:Mucins / Q000494:pharmacology; D009113:Muramidase / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D013666:Tears / Q000737:chemistry*; D013696:Temperature; D014835:Volatilization / Q000187:drug effects; D014885:Waxes / Q000494:pharmacology
chemical_list: D005136:Eye Proteins; D006838:Hydrocarbons; D007782:Lactoglobulins; D008055:Lipids; D009077:Mucins; D014885:Waxes; C042434:tear proteins; D009113:Muramidase