◆ developing foetuses, infants, and very young children;
◆ the elderly;
◆ groups of people with genetic polymorphisms;
◆ those who are socially and economically deprived.
7.3
Source-pathway-receptor model
Three essential factors determine the risk from a hazardous chemical present in the envi-ronment to human health. These are (i) the source of the chemical of concern, (ii) the pathway by which it can come into contact with the public, including air, water, land, and food, and (ii) the receptor — in this case the person or group of people.
Other factors which must be taken into account are:
◆ age at time of exposure, e.g. infancy, childhood;
◆ duration of exposure: brief, intermittent, or over a long period of time;
◆ level of exposure: a high or low dose;
This is simplified into the source-pathway-receptor model (Figure 7.1 ).
7.4
Increased susceptibility of developing foetuses, infants, and very young children
In 2009, the HPA published A Children’s Environment and Health Strategy for the UK at the request of the Department of Health, which clearly set out the unique aspects of envi-ronmental and other influences on health faced by children (Health Protection Agency 2009 ). Children are considered to be particularly susceptible to environmental chemicals compared to the general population of adults because of their fundamental differences from adults, which may lead to an unusual pattern of exposure (see Table 7.2 ). Young children also spend the majority of their time within the home, with one estimate of an aver-age of 19.3 hours per day in the UK (Farrow and Golding 1997 ). The home environment Table 7.1 Factors affecting vulnerability to an environmental hazard
Biological Sociocultural Ethnic
Age group (e.g. infant, elderly) Diet Genetic
Gender Smoking status Social (e.g. diet)
Disease state/medication Alcohol, drugs Genetic susceptibility Socioeconomic deprivation
Pregnancy (e.g. foetal development) Religion
Physiological variation (e.g. height, weight) Housing quality
Housing location
Occupation
Data from Risk assessment strategies in relation to population subgroups, Institute for Environment and Health, 1999 available from http://ieh.cranfield.ac.uk/ighrc/cr3.pdf (accessed 17.08.11)
Fig. 7.1 Illustrating the source-pathway-receptor model.
Hazard identification
• Nature of the hazard
• Location, quantity
Exposure assessment
• Direct, indirect
• Air, land, water, food
Risk characterisation
• Who is being exposed
• Time of exposure (child, adult)
• Duration of exposure
• Dose of exposure SOURCE
PATHWAY
RECEPTOR
is a unique environment known to accumulate air pollutants at a higher concentration than outdoors (Whitmore et al. 1994 ). Children also have a longer expected length of life left which all lead to a potentially greater exposure to toxic chemicals.
7.4.1 Physical differences
Children are much smaller than adults, which means that they tend to be in much closer contact with the ground. Proportionally they have a larger surface area of skin compared to their body size, which increases the potential for exposure and the absorption of chem-icals through their skin. Proportional to body weight, children breathe more air, drink more water and eat more food than adults do. The amount of air breathed in by a child while resting, per unit time on a weight-by-weight basis, may be nearly three times that of an adult.
7.4.2 Time windows
Children are both growing and developing rapidly, with their internal structures con-stantly developing and maturing. This is vital for the correct functioning of their bodies, for example their ability to produce specific hormones. Exposure during critical ‘time windows’ in development may cause irreparable damage and render the body unable to function properly. The organs that are particularly sensitive to chemical damage during development include the brain and central nervous system, the immune system, and the reproductive system.
7.4.3 Metabolic pathways
Some internal functions of infants and young children differ from adults, making children more vulnerable to exposure. Their mechanisms for detoxifying and excreting environmental
chemicals are immature and less efficient. Metabolic pathways may not yet include the same enzymes, or the same amount of a particular enzyme, which an adult would use to metabolize and detoxify the chemicals that enter their bodies (Eskenazi et al. 1999 ). This means that a dose of a specific chemical, which an adult body could quickly eliminate before damage is caused, has the potential to cause harm to a child.
7.4.4 Behavioural patterns
Younger children have age-specific behaviours that may increase their exposure to haz-ardous chemicals. For example, playing close to the ground and crawling can increase potential exposure through the skin. Children also have a higher ratio of skin surface area to body weight compared to adults. Younger children routinely explore their environ-ment by putting fingers, toys, and other objects into their mouths, and contact with floors, carpets, lawns, and other surfaces during crawling may lead to enhanced exposure via hand-to-mouth and object-to-mouth transfer. Some children exhibit pica behaviour, which is the deliberate ingestion of non-food items.
Table 7.2 Differences between children and adults Factor How children differ from adults
Exposure to hazard Sources of exposure (e.g. industry, use in gardens) Pathways of exposure (e.g. dust, water, soil)
Routes of exposure (e.g. dermal, inhalation, ingestion) Greater intake of air/food/water per unit body Increased surface area to weight ratio Physiological factors Greater rate of circulation
Higher cell growth in many organs
Shorter height
Lighter weight
Faster rate of respiration
Pharmacokinetics Greater intake through the gut in the very young Higher rate of intake through the lungs
Reduced ability to break down harmful chemicals in the very young Higher membrane permeability affecting oral absorption of chemicals in the
very young
Undeveloped ability to bind and store chemicals Increased bioavailability in body
Decreased excretion from the body in the very young Pharmacodynamics Immature immune system
Different extent of effect and response to toxic substances in the very young (may be increased or decreased sensitivity depending on substance) Increased sensitivity of particular organs