Child?s Development and Respiratory System Toxicity

Abstract

Growth in Childhood Humans grow slower than many other species. A lot of growth and weight gain happen during childhood, and specifically during the first two years of age. Humans reach 70% of their adulthood height at the age of six. Growth accelerates during certain periods of childhood known as growth spurts. These growth spurts happen six times through childhood which are; neonatal, infantile, early childhood, middle childhood, late childhood, and pubertal period [1]. Yet, growth during each of these growth peaks varies remarkably among individuals, mainly due to the integrative and mixed influence of cultural, behavioral, nutritional, genetic, and environmental factors. General body growth (i.e. weight and height) differs from internal organs growth. Human organs grow at different rates because cell divisions in these organs vary in time and pattern. While the liver and kidneys grow fast throughout childhood, the brain has shown minimal weight increase from childhood to adulthood. Growth of adipose tissues and muscles in childhood is not as fast as it is in teenage and adult years. The Lungs grow and gain weight faster than the kidneys, but slower than the liver [2,3]. Physiological and Anatomical Development Children breathe at a faster rate than adults. This is because the number of alveoli in infants and children are less than those in Adults. Alveoli in children are also smaller in size than adults; therefore children need higher ventilation rate to meet the oxygen requirement of the body. With normal growth during the first few years of life, the size and weight of the lung increases, the ratio of ventilation rate to lung surface is relatively larger in infants and children compared to adults. Higher ventilation rate in children makes particles move faster along the respiratory tract and spend less time in any designated area of the lung. This faster movement and less residential time allow these particles to penetrate deeper in the lung, representing them more into the alveolar blood: air barrier [4]. The deeper penetration increases the dose absorbed in children. Other factors which assist such penetration are cardiac output, physical activity, and mouth to nose breathing ratio. Higher mouth to nose breathing ratio raises the chance of a deeper penetration because the mouth has a smaller surface area for the deposition of particles, and thus, more particles move into the lungs compared to particles coming in through the nose [4]. Physical activity in children is another factor contributing to increased ventilation rate which increases the inhalation of particles and vapors. In addition to that, physical activity increases mouth breathing during moderate to heavy exercises. One study found increased mouth breathing by 20% in children aged 8-16 years while performing a physical activity [5]. It's not yet clear when switching from nose to mouth breathing during exercise happens in children. However, it was found that switching in adults happens when inhalational rate reaches 15 L/min [4]. The different patterns of nasal structural development among children also affect the inhalation and deposition rate of particles. The structure of the nose in children allows lower nasal filtration rate and higher nasal airway resistance. These two factors increase mouth to nose breathing ratio and further the penetration of particles into deeper parts of the lower respiratory tracts [5,6], and probably cause a higher nasal deposition rate [7]. Deposition also varies with variations in breathing patterns. Variations of breathing patterns include fluctuations in tidal volume and breathing rates during childhood [8,9]. Cardiac output decreases as age increases; adults have a lower cardiac output compared to children. Ventilation rate, size of the body, metabolic rate, and physical activity are factors affecting the cardiac output. All of these variables are higher in children compared to adults, which leads to a higher cardiac output. A higher cardiac output allows for rapid delivery and exchange of substances between the blood and different body tissues. This delivery is enhanced for the free unbounded circulating chemicals, which are not bound to plasma binding proteins. This too is higher in children in comparison to adults due to the lower amounts of such binding proteins as well as their functional immaturity [10]. Chemicals that don't bind or conjugate to any plasma proteins are either metabolized or eliminated. Such elimination processes are less efficient in children due to their lower glomerular filtration rate and lower renal clearance efficiency in the first six months of age [11].