and to miss work (Parker et al., 1994a). Inasmuch as back pain is rare among adolescents, and history of back pain has been identified as a risk factor for new back injuries (Mitchell et al., 1994; Venning et al., 1987), the long-term consequences of back strains among adolescent workers are of substantial concern.

Besides being vulnerable because of their rapid growth, children and adolescents may be at risk because of mismatches between their size and the dimension of equipment or machinery designed for adults. For example, the Consumer Product Safety Commission found that young, short, and light-weight operators of ride-on mowers were more likely than others to be injured (unpublished 1993 Consumer Product Safety Commission data, as cited in National Institute for Occupational Safety and Health, 1997). Specifically, operators were at increased risk of injury if their height was less than 60 inches, weight was less than 125 pounds, or their age was less than 15. Growth charts suggest that about half of all 15-year-olds weigh less than 125 pounds. The association of small body size with injuries from ride-on mowers raises concerns about the operation of other machinery by children and young teens.

Development of the Endoctine System

Puberty is central to the normal development that occurs in adolescence. The biological systems that lead to reproductive capacity are initiated and maintained by a complex system of hormones in the brain and reproductive organs. Although the time of onset and the speed with which each stage occurs vary widely among individuals, the events that mark puberty for both girls and boys occur in a predictable sequence. Although there are no data to demonstrate adverse effects on normal hormonal development, there are concerns that any chemical exposures that alter the delicate balance of these hormones and their feedback loops could have devastating effects, given the importance of the endocrine system during adolescence.

Cognitive and Behavioral Development

A number of changes occur in cognitive abilities as an individual moves from childhood through adolescence and into adulthood. In general, from early adolescence on, "thinking tends to involve abstract rather than merely concrete representation; to become multidimensional rather than limited to a single issue; to become relative rather than absolute in the conception of knowledge; and to become self-reflective and



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