in response to stimuli in the elderly. These changes likely represent the combined affect of repetitive antigenic stimuli from environmental exposure and age-related decline in down regulatory response to antigenic exposure. Persistent low grade inflammation in the lower respiratory tract can cause proteolytic and oxidant-mediated injury to the lung matrix resulting in loss of alveolar unit and impaired gas exchange across the alveolar membrane seen with aging. The clinical implication of immune dysregulation with age is yet to be determined. The cellular profile of epithelial lining fluid (ELF) changes with age. The ELF is rich in antioxidant defenses and minimizes oxidative injury to the respiratory epithelium following toxic exposure. ELF is rich in superoxide dismutase, catalase, metal binding proteins, glutathione, and vitamins C and E. In vitro and in vivo studies showed reduction in antioxidant levels in the ELF on exposure to ozone, nitrous oxide, and particulate matter. Composition of ELF changes with age, increasing susceptibility of older individuals to environmental toxic exposure (Kelly et al 2003) (Figure 1). Lung function Lung function can be divided into three categories: spirometry to assess the dynamic flow rates: forced expiratory volume in one second (FEV1), forced vital capacity (FVC), and FEV-1/FVC ratio. The dynamic flow rates are dependant on lung volumes (Table 2). The static lung volumes include total lung capacity (TLC), vital capacity (VC), residual volume (RV), and functional residual capacity (FRC) (Figure 3). The gas exchange across alveolar capillary membrane is measured using diffusion capacity for carbon monoxide (DLCO). Pulmonary function tests are reported as % predicted compared with individuals of same age, sex, and height. The lungs undergo a phase of growth and maturation during the first two decades of life and achieve maximal lung function around age 20 years in females and 25 years Table 2 Static lung volumes Tidal volume (TV) Inspiratory reserve volume (IRV) Expiratory reserve volume (ERV) Residual volume (RV) Clinical Interventions in Aging 2006:1(3) Volume of air inspired or expired during quiet breathing Maximum volume of air inspired above the tidal volume Maximum volume of air expired below the tidal volume Amount of air in the lungs after maximum expiration Aging and respiratory system Environmental toxin CD4+/CD8 Neutrophils Release of superoxide Damage lung matrix Air space dilatation and impaired gas exchange Figure 1 Effect of environmental exposure on airways. in males. Lung function remains steady with very minimal change from age 20 to 35 years and starts declining thereafter (Figure 3). The decline in pulmonary function tests depends on peak lung function achieved during adulthood, the duration of the plateau phase, and rate of lung function decline. Studies on lung function are done either to establish the reference values for the pulmonary function laboratories or to determine the age-related decline (Knudson 1981; Janssens et al 1999; Zeleznik 2003). The variability in similar physiologic measurements is much greater among healthy older individuals compared with younger individuals, making it problematical to establish a “normal” range for the older adults. Most prediction equations are derived from cross-sectional studies with under representation from older adults and ethnic minorities. The cohort and period effect unique to the cross sectional studies can under- or overestimate the lung function. Environmental exposures, nutritional deficiencies, or childhood infection common to individuals born within 255