Asthma patients  Inclusion criteria  Adults aged 18 years or over. Physician diagnosis of stable asthma (BTS guideline Step 3 or above).  Exclusion criteria  Recent asthma exacerbation requiring steroids and/or antibiotics in the 6 weeks prior to a study visit. Asthma deemed by a physician to be unstable or in an active treatment modification phase. Pregnancy (determined by urine pregnancy test for pre-menopausal women at the screening (V1) or eligibility visits (V3, V5). Resting blood oxygen saturations (SaO2) < 90% in room air, as measured by oximeter. Inability to comfortably lie supine. Any contraindication(s) to MRI scanning as per the MRI Screening Form and standard operating procedure used by the Unit of Academic Radiology, Sheffield. Disease or co-morbidity of sufficient severity as to make survival until 6 year follow up unlikely, as deemed by study clinician.  COPD patients  Inclusion criteria  Adults aged 18 years or over. Stage 3 or stage 4 COPD as defined by GOLD guidelines 2018, (https://goldcopd.org/wp-content/uploads/2018/11/GOLD-2019-v1.7-FINAL-14Nov2018-WMS.pdf) . FEV1:FVC <0.7 (spirometry results). FEV1 < 50% predicted (spirometry results). N.B. spirometry results that have been performed as part of a patient's normal clinical care within 6 months of study visit 1 may be used for definition of disease status and inclusion into the study; alternatively if no spirometry result is available it will be performed at visit 1.  Exclusion criteria  Recent COPD exacerbation requiring steroids and/or antibiotics in the 6 weeks prior to a study visit. COPD deemed by a physician to be unstable or in an active treatment modification phase, including the active phase of pulmonary rehabilitation. Pregnancy (determined by urine pregnancy test for pre-menopausal women at the screening (V1) or eligibility visits (V3, V5). Significant heart failure as assessed by clinician. Resting blood oxygen saturations (SaO2) < 90% in room air, as measured by oximeter. Inability to comfortably lie supine. Any contraindication(s) to MRI scanning as per the MRI questionnaire and standard operating procedure used by the Unit of Academic Radiology, Sheffield. Disease or co-morbidity of sufficient severity as to make survival until 6 year follow up unlikely, as deemed by study clinician.  ICU COVID-19  Inclusion criteria  Adult over 18 Proven COVID-19 Hospitalised/previously hospitalised on ICU requiring CPAP or mechanical ventilation Able to give informed consent For currently hospitalised participants, or those with significant on-going respiratory problems - Tolerates test inhalation of non-polarised gas according to supervising clinicians' judgement AND SaO2 do not fall below 80%  Exclusion criteria  No previous diagnosed respiratory disease with the exception of mild asthma, defined as:  no oral steroids in past year -no hospital admissions last year max 500mcg per day BDP or 250mcg fluticasone or equivalent ICS Pregnancy (determined by urine pregnancy test for pre-menopausal women) Significant heart failure as assessed by clinician. Resting blood oxygen saturations (SaO2) < 90%, as measured by oximeter at the time of scanning. Inability to comfortably lie supine. Any contraindication(s) to MRI scanning or Gadolinium contrast agent as per the MRI questionnaire and standard operating procedure used by the MRI Unit.  6. Stage 4 or 5 chronic kidney disease, defined by creatinine clearance less than 30 mL/min as estimated by the Cockcroft-Gault equation (due to gadolinium contrast use). 7. Previous allergy to gadolinium contrast. 8. Significant co-morbidity to make enrolment unsuitable, as deemed by study clinician.  Moderate/Severe COVID-19  Inclusion criteria  Adult over 18 Proven COVID-19 Hospitalised/previously hospitalised with a diagnosis of pneumonia (chest X-ray or CT scan consistent with COVID-19 infection)  Whilst hospitalised had new onset oxygenation impairment defined as:  a. An SpO2 ≤93% on room air and/or requiring additional oxygen to maintain satisfactory oxygenation  Able to give informed consent For currently hospitalised participants, or those with significant on-going respiratory problems - Tolerates test inhalation of non-polarised gas according to supervising clinicians judgement AND SaO2 do not fall below 80%  Exclusion criteria  No previous diagnosed respiratory disease with the exception of mild asthma, defined as:  no oral steroids in past year -no hospital admissions last year max 500mcg per day BDP or 250mcg fluticasone or equivalent ICS Pregnancy (determined by urine pregnancy test for pre-menopausal women) Significant heart failure as assessed by clinician. Resting blood oxygen saturations (SaO2) < 90%, as measured by oximeter at time of MRI scanning. Inability to comfortably lie supine. Any contraindication(s) to MRI scanning or Gadolinium contrast agent as per the MRI questionnaire and standard operating procedure used by the MRI Unit. Stage 4 or 5 chronic kidney disease, defined by creatinine clearance less than 30 mL/min as estimated by the Cockcroft-Gault equation (due to gadolinium contrast use). Previous allergy to gadolinium contrast. Significant co-morbidity to make enrolment unsuitable, as deemed by study clinician.  Mild- Non-hospitalised  Inclusion criteria  Adult over 18 Proven COVID-19 Not hospitalised for COVID-19 Able to give informed consent  Exclusion criteria  No previous diagnosed respiratory disease with the exception of mild asthma, defined as:  no oral steroids in past year -no hospital admissions last year max 500mcg per day BDP or 250mcg fluticasone or equivalent ICS Pregnancy (determined by urine pregnancy test for pre-menopausal women) Significant heart failure as assessed by clinician. Resting blood oxygen saturations (SaO2) < 90%, as measured by oximeter. Inability to comfortably lie supine. Any contraindication(s) to MRI scanning or Gadolinium contrast agent as per the MRI questionnaire and standard operating procedure used by the MRI Unit. Stage 4 or 5 chronic kidney disease, defined by creatinine clearance less than 30 mL/min as estimated by the Cockcroft-Gault equation (due to gadolinium contrast use). Previous allergy to gadolinium contrast. Significant co-morbidity to make enrolment unsuitable, as deemed by study clinician.  Non-hospitalised symptomatic (long-COVID group)  Inclusion criteria  Adult over 18 Proven COVID-19 Not hospitalised for COVID-19 Able to give informed consent Attended secondary care clinic with on-going symptoms.  Exclusion criteria  No previous diagnosed respiratory disease with the exception of mild asthma, defined as:  no oral steroids in past year no hospital admissions last year max 500mcg per day BDP or 250mcg fluticasone or equivalent ICS Pregnancy (determined by urine pregnancy test for pre-menopausal women) Significant heart failure as assessed by clinician. Resting blood oxygen saturations (SaO2) < 90%, as measured by oximeter at the time of scanning. Inability to comfortably lie supine. Any contraindication(s) to MRI scanning or Gadolinium contrast agent as per the MRI questionnaire and standard operating procedure used by the MRI Unit. Stage 4 or 5 chronic kidney disease, defined by creatinine clearance less than 30 mL/min as estimated by the Cockcroft-Gault equation (due to gadolinium contrast use). Previous allergy to gadolinium contrast. Significant co-morbidity to make enrollment unsuitable, as deemed by study clinician.