One third of cases of clinical intellectual disability does not have any obvious aetiology. Ill-defined environmental factors' relate to this. The figure does not account for subclinical effects. For example, a general reduction of 5-10 IQ points due to lead exposure may never be detected because victims are not classed as 'mentally handicapped'. Nor does the figure reveal time-latent outcomes - the effects that our present environment may have on the brains of future generations.
Victims are often concentrated in particular communities. Tiny Bhutan suffers a 22 percent incidence of intellectual disability due to iodine and other micro-nutrient deficiencies which relate to land degradation. Pollution from decaying industries has been blamed for doubling the numbers of children needing special education in regions of the Czech Republic. The effects of high levels of pollutants are compounded by poverty, malnutrition and bad health.