Sprawl: The New Manifest Destiny?

Manganese is a naturally occurring element that constitutes approximately 0.1% of the earth's crust, and low levels of Mn in water, food, and air are ubiquitous. In certain geo-logic regions, long contact times between ground water and bedrock enriched in Mn can lead to locally high levels in the water [U.S. Environmental Protection Agency (EPA) 2004]. Mn is an essential nutrient, and ingestion from drinking water is assumed to represent a small proportion of total intake, typically contributing < 1% although this can rise to 20% depending on Mn concentration in the water (Loranger and Zayed 1995). The rest of Mn intake occurs through food. Mn is thought to be better absorbed from water than from food, which is why the U.S. EPA proposed a modifying factor of three for assessing exposure to Mn from drinking water for the reference dose (RfD) (U.S. EPA 2004). In certain conditions of exposure, Mn is a potent neurotoxicant (Mergler and Baldwin 1997). Our knowledge of the neurotoxic properties of Mn has emerged almost exclusively from inhalation exposure in the workplace , particularly in mining, ore crushing, ferro-Mn production, and welding. High exposure to airborne Mn has been associated with neurotoxic effects, with the worst cases displaying an extrapyramidal syndrome (man-ganism), characterized by gait dysfunction with a propensity to fall backward, postural instability, bradykinesia, rigidity, micrographia, masked facies, speech disturbances, and muscle tremors (Rodier 1955). Lower levels of exposure may produce neurobehavioral deficits involving motor and cognitive functions, as well as psychological perturbations (Iregren 1999; Levy and Nassetta 2003). The neuro-biochemical disturbances observed concurrently with clinical and subclinical effects of intoxication involve the striatal dopaminergic system, although indications of GABA (gamma-aminobutyric acid)-ergic and sero-toninergic imbalance have also been reported (Dobson et al. 2004; Eriksson et al. 1987). Only a few reports are available on the effects of exposure to Mn in water. A study conducted in Greece showed that adults > 50 years of age presented neurologic signs with increasing levels of Mn in drinking water (up to 2,000 µg/L) (Kondakis et al. 1989), but younger individuals were not included in this investigation. The reduced compensatory capacity of older persons' nervous systems has been proposed as an explanatory mechanism for their apparent vulnerability (Kondakis et al. 1989). However, Vieregge et al. (1995) did not observe differences when comparing neurologic status and neurotoxic symptoms in adults > 50 years of age drinking water containing Mn …