Gas exchange

The gas exchange was a condition experienced by members of the Raxacoricofallapatorian Slitheen family whilst disguised in human skin suit via a compression field, caused by skin compression. Accompanied by a foul smell and loud noise, the gas exchange was similar to the human condition of farting but, as Harriet Jones observed, it did "not just smell like a fart" but also like bad breath, a symptom of calcium decay, Raxacoricofallapatorians being a calcium-based species. (TV: Aliens of London/World War Three, Revenge of the Slitheen) A gas exchange unit was a piece of technology worn by Slitheen. (AUDIO: Madquake) They eventually solved this problem, as well as becoming able to fit into slimmer skin suits. (TV: The Lost Boy)

History
In March 2006, the Passameer-Day Slitheen wore bulky skin suits and so "farted" repeatedly. As Harriet Jones and Rose Tyler observed, these "farts" smelt like bad breath, a symptom of calcium decay. From this, the Ninth Doctor realised that the Raxacoricofallapatorian were calcium-based lifeforms and so were weak against acetic acid, leading him to instruct Jackie Tyler and Mickey Smith to use vinegar to defend themselves against Sip Fel-Fotch Passameer-Day Slitheen. (TV: Aliens of London/World War Three)

Disguised as Lord Mayor of Cardiff Margaret Blaine, Blon Fel-Fotch Passameer-Day Slitheen "farted" whilst sat next to Suzie Costello, which she attributed to stress. Disgusted, Suzie described the smell as "lethal" and suggested that "Margaret" change her diet. (AUDIO: Sync)

When Blon took Cathy Salt to the toilets to kill her, she noted her "little tummy" was "complaining" when she farted loudly. Soon after, when she farted after entering the toilet, Cathy remarked, "Sounds like we got here just in time." (TV: Boom Town)

In the 2000s, Glune Fex Fize Sharlaveer-Slam Slitheen and Kist Magg Thek Lutiven-Day Slitheen "farted" repeatedly whilst disguised as Park Vale Comprehensive School headmaster and science teacher, Greg Blakeman and Tim Jeffery respectively, much to the amusement of the students. (TV: Revenge of the Slitheen)

Three Slitheen using stolen Blathereen technology wore slimmer human skin suits and did not experience "farting". (TV: The Lost Boy, PROSE: The Lost Boy)

In 2009, a Slitheen disguised as Ambassador Rahnius, who arrived in Sarah Jane Smith's attic to apparently reward her and friends, was exposed by his uncontrollable farting, despite blaming it on his chair. (TV: From Raxacoricofallapatorius with Love)

Later that year, two Slitheen disguised as Dave and Chris wore bulky skin suits and experienced gas exchange. (TV: The Gift)

In the 2010s, disguised members of the clan Klep-Clotch Slitheen wore skin suits which experienced gas exchange. (AUDIO: Death on the Mile)

At a point after the death sentence applied to the Slitheen had been abolished, Rose Tyler caught the stench of halitosis off disguised Slitheen on MXQ1. (AUDIO: The Taste of Death)

Other references
The naked Bella Slitheen let off an exceptionally long and loud fart without her gas exchange unit, which she attributed to being "really really" excited at the prospect of a human hunt. (AUDIO: Madquake)