Most obviously, children are generally smaller than adults. They have thinner skin, less subcutaneous fat and tissue, lower blood volume, and a higher ratio of body surface area to body mass.² This means that children, and particularly infants, are more sensitive than adults to changes in body temperature and have an increased risk of hypothermia. Children have a higher metabolism and more active cell division as their bodies grow, and some organ-to-body-mass ratios are larger. They have higher respiratory rates than adults; on average, young children breathe twice as much per kilogram of body weight per day, while newborn babies breathe 3 times as rapidly.³

In addition to physical factors, children exhibit behaviors during development that can exacerbate their risk of adverse health effects during CBRN disasters. Children spend significantly more time engaging in higher energy physical activity than adults, increasing their heart and respiratory rates.^4,5 Young children, including those ages 0 to 8 years, spend 2 to 4 hours per day playing indoors,⁶ which raises respiratory intake by over 20% from sitting.⁷ Depending on the level of activity, outdoor respiratory rates can be even higher. Running children breathe 4 times faster than sitting children. On average, young American children spend 4 to 5 hours per day outdoors.⁶ Young children touch dirt, smooth surfaces, and objects to their mouths multiple times per hour.^8,9 These behaviors may all contribute to an increased risk of physical exposure to agents, toxins, and other hazards during CBRN incidents. In addition, depending on age and development, children may not have the communication skills, motor skills, or judgment to effectively move toward safety in a dangerous situation.^10,11 In a state of panic, young children may not move, or they may simply begin crying or screaming.¹²

Children are dependent on caregivers, whether parents or others, at all hours of the day. As a result, children have a high number of personal contacts in their household and through school and childcare. In fact, children aged 5 to 19 years maintain higher personal contact rates per day than any other age group, with children 10 to 14 years old having the highest contact rates.¹³ Children's complex social networks have important implications for the spread of contagious diseases. Furthermore, their dependence on caregivers to make informed healthcare decisions on their behalf creates challenges. Many medical countermeasures (MCMs) are rarely tested or approved for use in children, and some can be administered only under regulatory mechanisms such as an Emergency Use Authorization or an Investigational New Drug protocol. Abiding by these regulations, providing the necessary information to receive informed consent from caregivers, and trying to maximize MCM coverage in a pediatric population during a large-scale event will be difficult. While plans exist to conduct studies to evaluate the safety of MCMs in children, the process to collect these data will be long.¹⁴

Biological agent exposure holds unique risks for children.^6,10 Young children spend approximately 4 to 5 hours per day outside, a particular concern with the airborne release of biological agents, such as Francisella tularensis, viral hemorrhagic fevers, or variola virus, which can survive from hours to days in the environment.¹⁵ Bacillus anthracis spores will remain a viable inhalation risk for many hours while descending before settling on the ground after an airborne release,¹⁶ and they can persist in the soil for centuries.¹⁷ Given how spores of hardier organisms like B. anthracis spread, infiltrate indoors, and potentially reaerosolize after a release,^18–20 infants and toddlers crawling or playing on the ground could be at a higher risk of infection through multiple routes of exposure. A child's developmental level can also play a role in route of exposure, as younger children have frequent hand-to-mouth behavior. Studies show soil ingestion by young children in contaminated areas leads to higher intake of toxic substances than by older children.²¹

For contagious agents, household contacts are at the highest risk of secondary infection. Children, particularly young children or children with disabilities, require a high number of personal contacts for daily care. In epidemic models, a disease incidence peak forms in school-age children, with a smaller peak in middle-aged adults (parents), for this reason.¹³ Smallpox has a nonspecific, noncontagious phase that confines an infected individual to home and bed rest. When the illness progresses to a more severe, contagious state, it raises the risk of infection of close contacts in the household. According to the WHO smallpox eradication campaign, males aged 5 to 20 were deemed “important in extrafamilial spread, since they moved between family groups more frequently and readily than other segments of the population.”²² Breastfeeding and other close contacts can expose infants to infection from biological agents or as a consequence of treatment—for example, the development of cutaneous anthrax infection after close contact with a mother's untreated lesion²³ or inoculation with vaccinia virus while breastfeeding.²⁴ In fact, 62% of secondary vaccinia infections in the US during pre-1972 routine vaccination occurred in children under 5 years of age.²⁵

Screening children for infection after they are exposed to biological pathogens is another challenge clinicians must face. Evidence-based screening algorithms are necessary for rapid triage and risk stratification. Adult screening criteria have been shown to be of limited value for evaluating children for anthrax infection.²⁶ Young children may have unusual presentations of disease. An infant was infected with anthrax during the 2001 US anthrax attacks and displayed symptoms unusual for cutaneous infection, thus delaying diagnosis.²⁷ Young children may also have difficulty describing symptoms, particularly symptoms such as difficulty breathing, chest discomfort, muscle pain, nausea, and headache. Diseases caused by other biological agents, such as pneumonic plague, smallpox, and tularemia, may present as nonspecific febrile illnesses. Acute febrile illnesses are common in young children, promoting a low index of suspicion among clinicians of infection with bioterror agents. Acute care in surge settings is further complicated by children's tendency to appear well and then decompensate rapidly when they are sick.

Because of their physiology, children are particularly susceptible to the effects of severe illness. Children are vulnerable to fluid loss, a risk exacerbated by the hemorrhagic or diarrheal symptoms of many biological agents. Children have a higher metabolic rate, increasing their susceptibility to inhaled toxins. Dose-response activity of anthrax is not well understood, but animal data suggest that duration of infection until death may correlate with body weight.²⁸ Smaller body size also requires alternative dosing for medical countermeasures, as it is not recommended that children take a full adult dose. Many children cannot swallow pills; for postexposure prophylaxis against anthrax, the Food and Drug Administration (FDA) recommends crushing doxycycline tablets for children weighing less than 90 pounds,²⁹ which amounts to approximately 14% of the total US population.^30–32 The need for crushable pills, liquid suspensions, or another alternative dosing methods limits the types of medications children can take in an emergency.

Release of radioactive materials into the environment may happen in a variety of both unintentional and intentional ways. Japan's 2011 Fukushima disaster is just the most recent example illustrating the need for radiological emergency preparedness in countries with nuclear power. Terrorist use of a radiological dispersal device (be it explosive or nonexplosive) and the detonation of an improvised nuclear device remain concerns.

Explosive radiological dispersal devices (“dirty bombs”) and nuclear weapons represent a significant hazard. The danger to children from blast injuries—primary injuries from over-pressurization force (blast wave) that have an impact on the body surface; secondary injuries from projectiles; tertiary injuries from being thrown into other objects; quaternary injuries like crush injuries, burns, and toxic exposures³³—is a consequence of several physiological factors. Thinner skin and a high body-surface-area-to-mass ratio make children particularly susceptible to the effects of blast and burn injuries. Open wounds from projectiles and burns coupled with acute radiation exposure lead to higher mortality in all populations from bone marrow stem cell damage, slow wound healing, and an ensuing high risk of secondary infection.^34,35 Combined injury with radiation exposure is a concern given children's risk of injury from blast and burn.

Beyond the immediate catastrophic damage and blast injuries that accompany a nuclear detonation, there is also the danger of exposure to ionizing radiation from the detonation and the resulting radioactive fission products. These fission products can travel long distances in the air plume, resulting in direct exposure to those below the plume and inhalation exposure to those inside it. Multiple exposure routes are possible when fission products settle on the ground as fallout. An improvised nuclear device detonated at ground level (known as a “ground burst”) is capable of sending large amounts of fallout into the atmosphere. Children are at risk for becoming contaminated with radioactive material above background levels, both internally (ie, through inhalation and ingestion) as well as externally on their skin and clothing. When children play on the floor, for example, radioactive particle resuspension rates are 500% higher than in undisturbed areas.³⁶ In the post-plume phase of an event, children can also receive a higher dose of external radiation from the same environmental exposure than adults, because they are closer to the ground and their organs are closer to the radiation source. And their internal radiation dose from ingestion of fallout and inhalation of resuspended fallout can be higher because of children's higher respiratory rates, hand-to-mouth behaviors, and dietary habits (overall greater consumption of milk, in which radionuclides concentrate, than adults). After the disaster in Chernobyl, the FDA developed derived interventional levels (DILs) to provide radionuclide-specific concentration limits in food by age group, protecting sensitive individuals such as children from exposure through dietary intake.^37–39

The behavior and developmental status of children are important determinants in their exposure to radioactive contamination; however, it is children's physiology that makes them particularly susceptible to adverse radiological health effects. The United Nations estimates that children carry a lifetime risk of cancer that is 2 to 3 times greater than that of the general population for a given radiation dose.⁴⁰ Children are considered to be at higher risk of breast cancer, brain cancer, thyroid cancer, non-melanoma skin cancer, and leukemia based on age at exposure. Children's growing bodies, more active cell division, greater milk consumption, and longer remaining life spans in which to express damage compared to adults lead to potentially greater radiation doses and increased lifetime cancer risk.² One such nuclide, radioactive iodine (I-131), can lead to thyroid cancer at high radiation doses. Other potential health effects include growth retardation, severe hypothyroidism, and thyroid gland nodules and neoplasms. These health effects were observed in children after the 1954 Castle BRAVO nuclear bomb test in the Marshall Islands.² Children's thyroid glands hold iodine concentrations that are 3 to 5 times higher than those in adults.⁴¹ In a radiation emergency, I-131 will accumulate together with stable iodine in the thyroid, leading to an absorbed radiation dose 8 to 9 times higher in infants than in adults.⁴⁰

Childhood and adolescent thyroid cancer incident rates increased markedly in the affected areas after the 1945 atomic bombings in Japan, the 1954 Castle BRAVO test, and after the 1986 nuclear power plant disaster in Chernobyl, Ukraine.⁴³ Nursing infants in the Marshall Islands received internal I-131 exposure from ingesting contaminated breast milk.² However, such cancer rates did not occur following the Three Mile Island (TMI) nuclear power plant accident in Pennsylvania.⁴³ This was attributed to stopping the consumption of locally produced milk, an effective public health action that did not occur in the Ukraine.⁴⁴

Children are also at high risk of adverse health effects from exposure to radionuclides having much longer half-lives than I-131. Radioactive cesium isotopes Cs-134 and Cs-137 can remain on the ground for many years.⁴⁵ As with I-131, there have been historical examples of transfer of Cs-137 from mothers to their children through breast milk.⁴⁶ The prevailing winds blew most of the radioactivity from Fukushima out to sea, but limited amounts of fission products (particularly strontium-90 [Sr-90], I-131, Cs-134, and Cs-137) were blown inland across populated areas,⁴⁷ and high levels of these isotopes were recorded in rainfall, tap water, vegetation, and in the milk supply worldwide.⁴⁸ Affected populations in Japan (those living in affected areas who may have been exposed to and possibly contaminated with these radionuclides) are being monitored for long-term health effects; a large cohort representing all age groups will receive lifelong cancer screening services.⁴⁹ Sr-90, a fission product present in fallout, behaves chemically like calcium. It is absorbed from the gastrointestinal tract at a slightly higher rate in children and accumulates faster in growing bones, posing an increased risk to children.⁵⁰ Bone growth occurs in spurts. Infants and adolescents characteristically experience the most rapid growth of bone and are therefore most susceptible to the hazards posed by incorporation of radioactive strontium into the bone matrix.⁴⁰

Radioactive elements need not be internalized in or deposited on the body to pose a health threat. Radiation exposure from nonfission radionuclides that could be used in a dirty bomb or nonexplosive radiological dispersion device (RDD), such as cobalt-60 (Co-60), holds risks for children as well. Co-60, a high-energy gamma source, is used commercially in various settings, including cancer treatment, and was recently involved in a theft in Mexico.⁵¹ Few studies of the effects of Co-60 in children exist, but data in Taiwanese children following exposure suggest that boys raised in Co-60–contaminated apartments experienced less growth relative to their peers.⁵²

While chemical agents are unlikely to affect as large a population as biological or nuclear weapons, they present significant challenges for children. Multiple reports of the August 2013 chemical weapons attack in Syria count a high percentage of children among the casualties,⁵³ with the American assessment counting 426 children out of 1,429 fatalities.⁵⁴ Young children with an underdeveloped sense of self-preservation may not run away from a cloud of white gas, even if others are doing so. Unlike most CBRN agents, some nerve agents can be absorbed through intact skin, leading to more severe exposure for young children who have more permeable skin and higher body-surface-area-to-mass ratio. Pulmonary agents like chlorine gas can be especially destructive to children, as children have narrower airways and twice the metabolic oxygen requirement as adults.⁵⁵ In addition, certain chemical agents, such as chlorine and sarin, are heavier than air, causing them to settle nearer to the ground where they are more easily inhaled by children. Children's higher respiratory and metabolic rates put them at higher risk for intake of these toxins. Additional considerations for children after a chemical attack include their requirement for smaller proportioned clothing after decontamination and the danger of hypothermia in a water-based decontamination effort.