Looking on the internet deeply has found these results:

I work in ER is the most popular phrase on the web.

I work at ER 123,000 results on the web	I work in ER 163,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
I don't work in the ER. Why don't they just hire trained monkeys to work the ER? So, I understand you work in the ER. Er... what did I put down? Well, obviously, my extensive charity work, er... I work graveyard as an ER nurse, and when I get home, I pretty much collapse. He used to work in, er, Ministry of Agriculture. Came back from work and er... she told me to pack my stuff. I used to work in the ER at Country General.	Er... what did I put down? Well, obviously, my extensive charity work, er... I don't work in the ER. So, I understand you work in the ER. Came back from work and er... she told me to pack my stuff. I used to work in the ER at Country General. I'm sorry, I don't mean to be callous, but when you work in the ER, you don't remember names. He used to work in, er, Ministry of Agriculture. I work graveyard as an ER nurse, and when I get home, I pretty much collapse.

I work at ER

123,000 results on the web

I work in ER

163,000 results on the web

More popular!

Er... what did I put down? Well, obviously, my extensive charity work, er...
I don't work in the ER.
So, I understand you work in the ER.
Came back from work and er... she told me to pack my stuff.
I used to work in the ER at Country General.
I'm sorry, I don't mean to be callous, but when you work in the ER, you don't remember names.
He used to work in, er, Ministry of Agriculture.
I work graveyard as an ER nurse, and when I get home, I pretty much collapse.