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This reader question regarding Social Security disability arrived by email today:

I am in the process of gathering info so I can decide if I should apply for disability.  It appears that the first criteria to meet is to not be working.  I am working and it is very, very difficult.  Will I need to give up my job before beginning this process in order to be eligible?

My answer:

Pretty much, yes. If you are working and earning more than $980 per month, then you are not disabled under Social Security's rules. There are a few exceptions to this, but in general if you are working and earning more than the level of substantial gainful activity ($980/month in 2009), then you are not disabled. Period. 

That determination is step 1 of Social Security's sequential evaluation process. If you are earning more than SGA-level wages, then the SSA will not even reach the issue of your impairments at step 2.
You didn't say what kind of work you do, or if you get a salary or hourly wages. However, if full-time work becomes too difficult for you, and you drop down to part-time work and earn less than $980/month, then you would be eligible to apply for SSDI benefits.