The goal of this clinical trial is to see if several weeks of self-administered,home-based, treatment involving breathing hydroxy gas (a mixture of hydrogen and oxygen)for at least 2 hours a day for 3 weeks, will relieve symptoms in patients suffering fromLong COVID. The main question it aims to answer is whether inhaling hydroxy gas might bea useful treatment option to help patients with long COVID cope better and recoverquicker from this condition.Participants will wear a nasal canula (placed in their nostrils) to inhale a gas from amachine that they will be trained to use at home. In one 3-week period, the machine willdeliver hydroxy gas (treatment) and in a separate 3-week period the machine will delivernormal air (placebo). The order of the treatment or placebo periods will be randomizedand separated by a minimum of 3-weeks during which the participants will not use themachine ('washout' period). Neither the participant nor the investigators will know which3-week period is the treatment and which is the placebo phase. Participants will visitthe laboratory (or be tested at home) at the start and end of each 3-week period.Testing will involve measuring physical ability (handgrip strength, how far they can walkin 6 minutes, how many times they can stand up and sit down in a minute), breathingproblems (how hard they can blow out, how breathless they feel), cognitive ability (howquickly they can mark out a trail based on numbers and letters), and state of mind(mood).The investigators hypothesize that compared to inhaling placebo, inhaling the hydroxy gaswill produce greater improvement in physical ability, relieve breathing problems, andenhance cognitive ability and mood, thereby showing that it can relieve key symptoms oflong COVID