Why does Chuck ask people that enter his house if they have 'grounded themselves'?

I found out that this is called Electromagnetic hypersensitivity. To put it simple, people are affected by electromagnetic fields. They show many symptoms, like pain and ache, headaches etc.

Per wikipedia:

Electromagnetic hypersensitivity (EHS) is a group of symptoms purportedly caused by exposure to electromagnetic fields [...]

The reported symptoms of EHS include headache, fatigue, stress, sleep disturbances, skin symptoms like prickling, burning sensations and rashes, pain and ache in muscles and many other health problems. Whatever their cause, EHS symptoms are a real and sometimes disabling problem for the affected person. However, there is no scientific basis to link EHS symptoms to electromagnetic field exposure.

Also, on the first link I posted above, it is mentioned that all of these could just be on Chuck's head.

?lso, this condition was mentioned on S01E05, where Jimmy oversimplified it by calling it 'allergy to electricity'.

Like @System pointed out in the comments, this isn't accepted as a diagnosis. The doctor at the show, seems to believe that this is a mental situation as well.

To be more specific than the existing answers: Chuck has this metal knob on his porch that people are supposed to touch before entering, to get rid of any static electricity they might be carrying. This is what he means with "grounding themselves".

It's been said in the episodes before. He's afraid of electromagnetic radiation. He has electromagnetic hypersensitivity (EHS).

Electromagnetic hypersensitivity (EHS) is a group of symptoms purportedly caused by exposure to electromagnetic fields.