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Table 2 Quality of overall evidence for each environmental factor

From: Environmental risk factors for dementia: a systematic review

Factor N Studiesa Overall Strength Of Evidenceb Direction Of Associationc
C X R
Air
 Nitrogen oxides (NOx) 2    Strong
 Carbon monoxide (CO) 1    Moderate
 Environmental tobacco smoke   1   Moderate
 Particulate matter (PM10&2.5) 1 1   Strong
 Ozone (O3) 1 1   Strong
Toxic heavy metals
 Arsenic   2   Moderate
 Lead   1   Weak
Other metals
 Aluminium 1 15   Moderate
 Calcium 1    Weak
 Cobalt   1   Weak
 Copper   2 1 Weak
 Iron   2 1 Weak
 Manganese   1   Weak
 Molybdenum   1   Weak
 Nickel   1   Weak
 Uranium   1   Weak
 Zinc   2   Weak
Other trace elements
 Fluoride   1   Weak
 Selenium    1 Moderate
 Silicon (and silica) 2 2   Strong
Occupational
 Aluminium (occupational exposure) 1 3   Weak
 Defoliants/fumigants 1    Weak
 Diesel motor exhaust 1    Moderate
 Electromagnetic fields 1    Moderate
 Excessive noise 1    Weak
 Glues/adhesives 1 1   Weak
 Pesticides/fertilizers/herbicides/insecticides 5 2 2 Strong
 Lead (occupational exposure)    1 Weak
 Metals (occupational exposure) 1    Moderate
 Inks/dyes 1    Weak
 Paints/stains/varnishes 1    Weak
 Gasoline/fuels/oils 1    Weak
 Solvents/degreasers 2 1 1 Strong
 Liquid plastics/rubbers 1    Weak
 Vibratory tools 1    Weak
 Radiation 1    Weak
Miscellaneous
 Climate   1   Weak
 Electric and magnetic fields 1   2 Moderate
 Mobile phone use 1    Weak
 Vitamin D 3 1   Strong
 Water pH   1   Weak
  1. aC cohort studies, X cross-sectional studies, R reviews. Studies can appear in multiple rows
  2. bStrength of evidence is assessed according to the criteria outlined in Table 1
  3. cIncreased levels of the exposure are associated with: ↑ an increased risk of dementia; ↓ a decreased risk of dementia; ↕ mixed results on dementia risk; and – no substantial effect on dementia risk