You’re being sarcastically dismissive, but this is a real possibility. The real world is complicated and disorders with environmental effects are often multi-factorial.
Air pollution might not be the direct cause, it might be a proxy measurement that is correlated with some other factor or factors that contribute to dementia risk. For example, do areas with higher air pollution measurements also have higher or lower rates of something else that is actually contributing to the dementia directly? Do they simply correlate with overall development of the area, and therefore areas with poor pollution numbers also have high levels of water pollution?
Air pollution might not be the direct cause, it might be a proxy measurement that is correlated with some other factor or factors that contribute to dementia risk. For example, do areas with higher air pollution measurements also have higher or lower rates of something else that is actually contributing to the dementia directly? Do they simply correlate with overall development of the area, and therefore areas with poor pollution numbers also have high levels of water pollution?