How Are Newborns, Infants, and Toddlers Exposed To and Affected by Toxicants?
Newborns and infants exhibit unique vulnerabilities to environmental toxicants. The growth rate during the first few months of life following birth is faster than during the rest of life. Tissues with rapidly dividing cells may be especially vulnerable to carcinogens. Vulnerable tissues include
- epithelium, and
Children’s growth velocity decreases smoothly at approximately 9 months of age – to about half the initial rate. Vulnerability to some toxicants such as nitrates decreases. How toxicants enter the body – the routes of exposure – will be considered in the context of some health effects in newborns, infants, and toddlers.
The small intestine of a developing child responds to nutritional needs by increasing the absorption of specific nutrients. For example, calcium transport in newborns and infants is about five times the rate in adults. If lead exposure occurs, the lead will compete with the calcium for transport at this high rate. Thus, children’s absorption of ingested lead may be five times higher than that of adults [NRC 1993b].
In most circumstances, breastfeeding is the optimal form of infant nutrition. Human milk provides advantages with regard to general health, growth, and development while significantly decreasing the child’s risk for a large number of acute and chronic diseases. Breastfeeding’s many benefits to the infant greatly outweigh any risk from possible contaminants in breast milk.
A breastfeeding baby, however, remains vulnerable to current and historic maternal exposures. Lactation mobilizes previously sequestered fat-soluble toxicants such as dioxins, polychlorinated biphenyls (PCBs), or chlorinated pesticides, which then contaminate breast milk [Solomon and Weiss 2002; Karmaus et al. 2001]. Maternal toxicokinetics – that is, the solubility and binding properties of a toxicant and the characteristics of breast milk – determine the milk-maternal plasma (M/P) ratio. The higher the ratio, the more complete the transfer of the substance into breast milk. Substances that transfer most readily include those that are
- low-molecular-weight, and
- highly lipophilic.
M/P ratios have been published for a variety of xenobiotics [Schreiber 2001]. The M/P ratio for lipophilic substances such as PCBs ranges from 4 to 10; the ratio for organic and inorganic mercury is 0.9.
On a daily basis, a newborn infant consumes a much larger amount of water (equivalent to 10%-15% of body weight) compared with an adult (2%-4% of body weight). Formula-fed infants consume significant amounts of water; average daily consumption might be 180 milliliter (mL)/kilogram (kg)/day (6 fluid ounces (fl oz)/kg/day). This is the equivalent of thirty-five 360-mL (12 fl oz) cans of soft drink per day for an average adult male [Paulson 2001].
Water from municipal water systems is usually low in lead. But the water can acquire lead from lead pipes connecting the water main to the home, or lead pipes or lead-soldered pipe joints in the home. The first-draw water (i.e., water that has stood overnight in pipes) should be discarded. Water contaminants such as lead (and nitrates) are concentrated when water is boiled. And in each area of the country, local and state authorities may have issued their own area-specific advisories.
Many families use private well water and consider it safe – perhaps safer than municipal water. Private well water, however, is largely unregulated and unmonitored. It has the potential for exposure to contaminants. For example, nitrate is a well-recognized hazard in well water.
Below are some of the reasons young infants are at increased risk of methemoglobinemia from nitrate exposure from well water
- Age-dependent changes in pH. The gastric pH of infants is higher for the first 12 months of life and does not drop to adult levels until 3 years of age [Marino 1991]. A high gastric pH leads to excess bacterial colonization, which increases nitrate-to-nitrite conversion.
- Age-dependent enzyme activity. NADH-dependent methemoglobin reductase activity in infants is 60% that of adults. The relative lack of the methemoglobin reductase enzyme needed to convert methemoglobin to functioning hemoglobin can lead to methemoglobinemia in young infants. At about 6 months, however, infants begin to reach adult levels of NADH-cytochrome b5 reductase, which converts methemoglobin to hemoglobin [Avery 1999].
A typical toddler’s diet is relatively rich in fruit, grains, and vegetables. Thus the exposure risk from foodborne pesticide residue is higher for toddlers than it is for adults, who routinely consume fewer of these foods per kilogram of body weight. The average child drinks 21 times more apple juice and 11 times more grape juice, and eats 2-7 times more grapes, bananas, carrots, and broccoli than does an average adult [NRC 1993a]. To lessen exposures to toxic chemicals, some regulations now acknowledge children’s different exposures and susceptibilities. For example, the Food Quality Protection Act of 1996 states that pesticide tolerance (the amount of residue legally allowed to remain on a food) must be set to protect the health of infants and children [EPA 2006].
The ratio of the newborn’s skin surface area to body weight is approximately three times greater than that of an adult [Guzelian et al. 1992]. Thus covering a similar percentage of the newborn’s body with a skin-absorbable substance will lead to a larger dose per unit of body weight compared with what an adult will absorb.
The dose is also affected by the surface area exposed and the vehicle, which may promote contact/residence time. In addition, skin characteristics of a newborn (birth to 2 months) enhance the absorption of xenobiotics [Mancini 2004]. During the fetal stage, the thick keratin layer that protects an adult’s skin from toxicants has not yet formed. Although this keratin layer begins to develop in the first 3-5 days after birth, it remains more permeable to absorption throughout the newborn period. As a result, the newborn skin more readily absorbs chemicals.
Absorption of environmental toxicants is inversely proportional to the integumentary thickness of the stratum corneum. As a result, young infants have more avid uptake of chemicals through their relatively thin epidermis than do older children and adults [Kearns et al. 2003]. Infants and children also have greater perfusion and hydration of the epidermis than do adults [Kearns et al. 2003]. This renders them more vulnerable to systemic effects of topical exposures, such as phenolic disinfectants (causing hyperbilirubinemia) [Wysowski et al. 1978] or iodine-containing antiseptics (causing hypothyroidism) [Clemens and Neumann 1989].
The younger the child, the higher the respiratory rate and the higher the weight-adjusted dose of an air pollutant. For example, newborns take an average 45 breaths per minute versus 31 breaths per minute for infants 6 months old, 24 breaths per minute for 2-year old toddlers, and 12-14 breaths per minute for adults [Gaultier 1985].
Respiratory system growth and development involves proliferation and differentiation of more than 40 cell types. An architecture branching over 25,000 terminations develops, giving rise to more than 300 million alveoli. This process is not complete until adolescence [Dietert et al. 2000]. The developing respiratory system may be more vulnerable to some airborne pollutants than the adult respiratory system.
A baby’s exposure to indoor and outdoor air pollution closely mirrors that of the parents or caregivers. But the greater vulnerability of the infant’s respiratory system increases the risk that early exposures to combustion air pollutants (e.g., secondhand smoke (SHS)) will slow the rate of pulmonary growth. Acute clinical effects in infants exposed to SHS include
- increased morbidity from respiratory syncytial virus (RSV) infection, and
- chronic middle ear effusions [Cook and Strachan 1999; Woolf 1997; Gitterman and Bearer 2001].
Respiratory exposures to air contaminants during the 1st year of life have a greater influence on the incidence and severity of asthma than do exposures later in life. Such air contaminants include
- cockroach antigens,
- dust mites,
- farm dusts and animals,
- oil smoke or exhausts,
- pesticides, and
- SHS [Etzel 2001; Belanger et al. 2003; Salam et al. 2004].
As infants and toddlers begin to explore the world away from the arms of parents or caregivers, they are often in the microenvironments* of the floor and ground. In these microenvironments, some toxic gases are heavier than air and layer close to the floor.
Examples of these toxic gases include
- aerosolized pesticides,
- carbon monoxide,
- mercury vapor, and radon.
The combination of a child’s high respiratory rate and- breathing zones close to the floor results in higher inhaled doses of toxicants than an adult would receive in the same room.
*A “microenvironment” is the environment of a small, specific area. This is in contrast to a “macroenvironment” – a larger area such as home, child care setting or school.
- Because of differences in absorption and the relatively larger amount of water in the diet, children may absorb more of a given substance in water and be exposed to greater doses than are adults.
- A newborn has a skin surface area three times greater by volume than does an adult. Thus the amount of a newborn’s skin a substance touches can result in a greater absorbed dose.
- Younger children have higher respiratory rates than adults. They therefore absorb more air contaminants per unit of weight: they therefore experience a higher inhaled dose.