My likely candidates: Fossil fuels. Rising CO2 levels can lead to brain fog, right now indoor CO2 levels due to bad ventilation already lead to imaired cognivity. With rising atmospheric CO2 levels this will become more common. Nano particles, different materials are being used today with dimensions similar to those of asbestos particles, and it turns out they have the same effect. It's not the asbestos, it's how the body handles these particles. Think of stuff like mineral wool, carbon nanotubes, silica. Sitting. More and more people are sitting down (inside, so they also lack vitamin D) for long parts of the day, and it leads to all kinds of medical problems.