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 from Long COVID. The main question it aims to answer is whether inhaling hydroxy gas might be a useful treatment option to help patients with long COVID cope better and recover quicker from this condition.

Participants will wear a nasal canula (placed in their nostrils) to inhale a gas from a machine that they will be trained to use at home. In one 3-week period, the machine will deliver hydroxy gas (treatment) and in a separate 3-week period the machine will deliver normal air (placebo). The order of the treatment or placebo periods will be randomized and separated by a minimum of 3-weeks during which the participants will not use the machine ('washout' period). Neither the participant nor the investigators will know which 3-week period is the treatment and which is the placebo phase. Participants will visit the 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 walk in 6 minutes, how many times they can stand up and sit down in a minute), breathing problems (how hard they can blow out, how breathless they feel), cognitive ability (how quickly 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 gas will produce greater improvement in physical ability, relieve breathing problems, and enhance cognitive ability and mood, thereby showing that it can relieve key symptoms of long COVID

Inclusion Criteria:

• Any moderate or severe pain, breathlessness, or fatigue associated with long COVID
• live within an hour's drive of the Oxford Brookes campus

Exclusion Criteria:

• Pre-existing secondary diseases; tumors, mental health conditions, chronic respiratory conditions such as Chronic Obstructive Pulmonary Disease (COPD), and cardiac disease
• Previous requirement of ICU treatment due to COVID
• Acute or chronic infections or no pre-existing autoimmune diseases

The study is double blind. The participants will not know whether the machine they use delivers hydroxy or placebo gas. The PI and outcome assessors will also not know which machine delivers hydroxy or placebo gas to participants during the running of the study. A collaborator will be in charge of blinding for the study.