They have evolved with a unique physiology in order to cope with this ultra-cold environment; although they breathe a fairly standard oxygen-nitrogen atmosphere at close to normal pressure, the Breen have no blood or other liquid circulatory system. Instead, their body is permeated by millions of tiny hollow tubules, which are open to the skin. These allow oxygen to be carried directly into the body; once there it is absorbed directly through the skin to be used by the internal organs. A similar process carries the waste gasses back out again. The Breen reproductive cycle is also unusual, in that they commonly produce offspring at a very early age. Given that most known major space faring species live on class M planets with an average temperature far above that of Breen, the Breen have had to develop environmental suits of great capability and reliability in order to co-exist with others. Given their nature, it is understandable that the Breen have become one of the Alpha Quadrants leading experts at creating and maintaining extremely cold environments. They are acknowledged experts on cold storage of chemical compounds.
Society: Given the environmental difficulties, casual visits to Breen worlds by non-Breen are virtually impossible. The Breen strongly discourage visitors, and all those who do visit Breen worlds are confined to purpose-designed encampments. Combined with the fact that the Breen habitually design environmental suits that hide their faces from view, this has given them their air of mystery. This has been further enhanced by the fact that some empathic species, most notably Betazoids, are unable to detect the thoughts or emotions of the Breen. Much of Breen is known to be a frozen wasteland, and it is widely thought that the Breen have deliberately avoided large surface developments for cultural reasons. Normal Breen body temperature is -38 C; exposure to temperatures above -13 C will kill them.
Politically nonaligned semi-humanoids from a world near the Black Cluster. The Breen normally wear heavy robes and an environmental support mask in a class M environment. Very little is known about their physiology or culture. Their technology is comparable to many other galactic powers. Although the Breen are politically nonaligned, many Breen act as pirates or raiders, jeopardizing their neutrality. The Breen can not be telepathically detected and have no such abilities themselves. Nevertheless, much has been discovered about the Breen over the years. They have developed organic technology to the point where they routinely use biological spacecraft. Some of these are fitted with cloaking devices, a technology the Breen apparently invented independently of the Romulans and Klingons. They use disruptor type weaponry rather than the phaser technology used by the Federation.
The Breen have something of an aggressive history; Breen privateers have conducted raids against other species on several occasions. In 2366 the Breen attacked and captured the Cardassian vessel Ravinok, using its crew as slave labor in the Dilithium mines on Dozaria. In 2372 the Breen also attacked the Bajoran colony of Free Haven. The Romulan saying, "Never turn your back on a Breen", sums up the reputation of this curious species. TNG "Hero Worship"5-111