Alvarez was removed from Friday's contest against the Braves in the fifth inning and immediately evaluated in the training room.
Astros manager Dusty Baker said Alvarez was experiencing shortness of breath caused by smoke from in-game fireworks, according to The Associated Press.
"I'm glad we got him out when we did because I looked up and he was in the dugout and it was kind of a scary moment because it could be anything," Baker said. "But they said he's doing fine at the moment."
Alvarez was released from hospital after all tests came back normal. He's at the ballpark Saturday and continues to undergo further evaluation.
The 25-year-old, who is having an MVP-caliber season, entered Friday slashing .297/.401/.616 with 31 home runs and 77 RBIs in 102 games.