'Our lungs are highly complex organs that are exquisitely specialized for gas exchange and host defense.' Rawlins (2010) 1. Introduction
General considerationsThe most important function of the lung is to acquire molecular oxygen and eliminate carbon dioxide. Except probably for the gastrointestinal system, no other organ in the body interacts with the external environment as constantly and as intimately as the respiratory system. For example, during a 24 hour period, at rest, the human lung is ventilated ~25,000 times with ~20,000 L of air (e.g. Burri, 1985; Brain, 1996), it has a respiratory surface area (RSA) of ~140 m 2 (about the size of a tennis court) which is located in the acini that lie no more than 40 to 50 cm from the external environment (air) (e.g. Weibel, 1984), and the thickness of the blood-gas (tissue) barrier (BGB) (harmonic mean thickness, ht) is 0.62 μm, a value about one-fiftieth of the thickness of a foolscap paper or that of a human head hair (Gehr et al., 1978(Gehr et al., , 1990a. By weight, each day, more than 20 kg of air enters and leaves the human body, a load that far exceeds that of food and water ingested during the same time period (Brain, 1996). Depending on the level of air pollution, different types and quantities of foreign particulates and microbial pathogens are inhaled. While large RSA and thin BGB increase gas exchange, the concomitant downside to these structural properties is that they make the lung a leading portal of entry and therefore attack by pathogenic micro-organisms, damage by allergens and particulates, and injury by noxious gases (e.g. Brain, , 1992Lambrecht et al. 2001;Garn et al., 2006). The inhaled particulates are deposited on the epithelial lining of the conducting airways and that of the peripheral air spaces where they are retained for various durations before they are removed or destroyed (e.g. Geiser et al., 1988; Gehr et al., 1990 a, b). Brain (1996) observed that 'the lung is unique in that the marriage between environment and lung disease is profound. The respiratory system threfore forms a huge challenge to the body's immune integrity (e.g. von Garnier and Nicod, 2009 , 1993;Schwartz, 1994;Brunekreef et al., 1995;Ware, 2000;Pope, 2000;Nemmar et al., 2002;Pope et al., 2002;Suwa et al., 2002;Schulz et al., 2005;Kaufman, 2010;Brook et al., 2010; van den Hooven et al., 2011;Kampfrath et al., 2011 gc.ca/ccdpc-cpcmc/crdmrc/facts_gen_e.html). According to the Canadian Lung Association (CLA), the economic burden of respiratory disease in the Country is ~3 billion ($US) dollars (http://www.bukisa.com/articles/455926_lung-health-occupational-healthincidence#ixzz1I4jwMvBX). Based on net changes in gross domestic product (GDP) growth forecasts, the estimated annual cost of SARS (Sudden Avian Respiratory Syndrome) in Asia exceeds 10 billion dollars ($US) (Lee and McKibbin, 2003;Fan, 2003;McKibbin and Sidorenko, 2006) and according to the World Bank's estimates, an influenza pandemic may result in a loss (expenditure) of 800 billion dollars ($US) (Brahm...