Nationals Park hosted boy band One Direction earlier this week, and the park decided to place the few dozen porta-potties in the team's bullpen.
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The Washington Post pointed out that this has been the case for each of the concerts held at the venue this season, with the team's bullpen dealing with the fallout the next day
Apparently, the Nats' bullpen still smelled like port-a-potties from the Jason Aldean concert. In every way, that game stunk. — Adam Kilgore (@AdamKilgoreWP) August 1, 2014
As for why the portable bathrooms are not at least placed in the visitors pen, the stadium officials told The Post it is simply a matter of logistics as the home 'pen is closer to the service tunnel.
If you are an opposing hitter, you should just give yourself the greenlight against the teams relievers after concerts.