Volus

The Volus are a sapient, ammonia-breathing species who hail from Irune, a world with a gravitational field 1.5 times that of the Earth, as well as a high-pressure atmosphere. The high-pressure atmosphere produces a greenhouse effect that aids in the support of its ammonia-based ecological biochemistry, but also results in the Volus' absolute need to wear pressurized environmental suits and breathers when dealing with other species, many if not most of which breathe oxygen and hail from planets of lower atmospheric pressures. They are an associate race of the Citadel Council with their own (albeit tiny) embassy, but they are also a client race of the Turians.

Biology
Thanks entirely to their homeworld's relatively unique conditions, the Volus are unable to survive unprotected in an atmosphere which is more suitable for Humans and other carbon-based lifeforms. This forces them to wear protective suits capable of providing the proper atmosphere and pressurization required to support their existence. Traditional nitrogen/oxygen air mixtures are in fact poisonous to them, and in the low pressure atmospheres tolerable to most other species, their flesh will actually split open.