title: Thoracic and respirable particle definitions for human health risk assessment.
abstract: BACKGROUND
  Particle size-selective sampling refers to the collection of particles of varying sizes that potentially reach and adversely affect specific regions of the respiratory tract. Thoracic and respirable fractions are defined as the fraction of inhaled particles capable of passing beyond the larynx and ciliated airways, respectively, during inhalation. In an attempt to afford greater protection to exposed individuals, current size-selective sampling criteria overestimate the population means of particle penetration into regions of the lower respiratory tract. The purpose of our analyses was to provide estimates of the thoracic and respirable fractions for adults and children during typical activities with both nasal and oral inhalation, that may be used in the design of experimental studies and interpretation of health effects evidence.
  
  
  METHODS
  We estimated the fraction of inhaled particles (0.5-20 μm aerodynamic diameter) penetrating beyond the larynx (based on experimental data) and ciliated airways (based on a mathematical model) for an adult male, adult female, and a 10 yr old child during typical daily activities and breathing patterns.
  
  
  RESULTS
  Our estimates show less penetration of coarse particulate matter into the thoracic and gas exchange regions of the respiratory tract than current size-selective criteria. Of the parameters we evaluated, particle penetration into the lower respiratory tract was most dependent on route of breathing. For typical activity levels and breathing habits, we estimated a 50% cut-size for the thoracic fraction at an aerodynamic diameter of around 3 μm in adults and 5 μm in children, whereas current ambient and occupational criteria suggest a 50% cut-size of 10 μm.
  
  
  CONCLUSIONS
  By design, current size-selective sample criteria overestimate the mass of particles generally expected to penetrate into the lower respiratory tract to provide protection for individuals who may breathe orally. We provide estimates of thoracic and respirable fractions for a variety of breathing habits and activities that may benefit the design of experimental studies and interpretation of particle size-specific health effects.
authors.affiliation: National Center for Environmental Assessment, US Environmental Protection Agency, MD B243-01, Research Triangle Park, Raleigh, NC 27711, USA. Brown.James@epa.gov
authors.forename: James S
authors.initials: JS
authors.lastname: Brown
authors.forename: Terry
authors.initials: T
authors.lastname: Gordon
authors.forename: Owen
authors.initials: O
authors.lastname: Price
authors.forename: Bahman
authors.initials: B
authors.lastname: Asgharian
mesh_terms: D000203:Activities of Daily Living; D000328:Adult; D000336:Aerosols; D000367:Age Factors; D000393:Air Pollutants / Q000009:adverse effects / Q000737:chemistry*; D001822:Body Burden; D002648:Child; D005260:Female; D006801:Humans; D019570:Inhalation Exposure / Q000009:adverse effects; D016014:Linear Models; D008297:Male; D008954:Models, Biological; D010316:Particle Size; D012119:Respiration; D012137:Respiratory System / Q000378:metabolism*; D018570:Risk Assessment; D012307:Risk Factors; D012737:Sex Factors; D009626:Terminology as Topic; D014116:Toxicology / Q000145:classification / Q000592:standards*; D055815:Young Adult
chemical_list: D000336:Aerosols; D000393:Air Pollutants