These include asthma, where the airways become inflamed and tight, and chronic obstructive pulmonary disease (COPD), an umbrella term for a group of conditions that make it difficult to empty air out of the lungs due to the airways becoming narrowed.
Although the causes of all respiratory diseases are unknown, some of the most common are smoking; harmful pollutants in the air, which can vary depending on the weather and season; and genetics. They can also be caused by exposure to chemicals at work, such as breathing in asbestos fibres which damages the lungs.
The exact cause of asthma is unknown, although you're more likely to develop it if: you have a family history of asthma, eczema or any allergies; had bronchiolitis as a child; were born prematurely; or your mother smoked during pregnancy.
Nine out of 10 cases of COPD are estimated to be caused by smoking. Long-term exposure to harmful chemicals, fumes or dust may also contribute. Two of the most common types of COPD are bronchitis, where the airways become inflamed; and emphysema, where there's damage to the air sacs in the lungs.
Other types include cystic fibrosis, an inherited disease where the lungs and digestive system become clogged with thick mucus; lung cancer; and pneumonia, where there's inflammation in one or both lungs caused by bacterial or viral infection.
According to a study, published by the Journal of the American Geriatrics Society, having a severe vitamin D deficiency may put people aged 65 and above at more than twice the risk of having respiratory disease.
These include having an unusual lack of energy, shortness of breath, tightness in the chest, coughing and wheezing.
Speak to your GP if you have persistent symptoms and they will ask if you smoke or have been exposed to dust or harmful chemicals. They may also carry out some breathing tests. A spirometry test measures the amount of air you can breathe in and out and its flow rate; a peak flow test, measures the rate at which you can force air out of your lungs, and may rule out asthma.
Further tests may include a blood test; a chest X-ray, which can highlight problems in the lungs; a computerised (CT) scan of the lungs; a hypoxic challenge (fitness-to-fly) test, which assesses how your lungs react to reduced oxygen; and a bronchoscopy, which looks inside your lungs by inserting a thin, flexible telescope. A small piece of lung tissue (biopsy) may also be taken.
Who gets respiratory diseases?
Smoking is a major risk factor for respiratory diseases, and in the UK around 25,000 deaths each year are attributed to COPD.
Around 5.4 million people in the UK are treated for asthma. It's more common in women than men but affects more boys than girls. Work-related respiratory diseases cause 13,000 deaths each year.
Treatment depends on the type of respiratory illness you have. It's important to stop smoking, especially with COPD, as it's the most effective way of preventing it getting worse.
If you have asthma you will be advised to reduce exposure to triggers that bring on attacks such as pollution and weather conditions. Drug treatments include 'preventer' inhalers, which contain inhaled steroids that stop asthma attacks occurring by reducing sensitivity, swelling and inflammation in the airways; and 'reliever' inhalers, which contain medication called short-acting beta agonists (SABAs), also known as salbutamol (Ventolin®). These work by relaxing the muscles around the airways and are only needed when symptoms start.
People with COPD can also be treated with inhalers. Other drug treatments include theophylline tablets to open the airways and mucolytic tablets to reduce phlegm. Antibiotics and steroids may also be prescribed for short courses.
Vitamin D supplements can also help with asthma symptoms. A review of studies in 2016 concluded that people with mild to moderate asthma who took vitamin D3 supplements had fewer severe attacks overall.
A 2016 study found older people in long-term care given high doses of vitamin D3 monthly were 40 per cent less likely to suffer from acute respiratory illnesses. Another study published in 2017 in the British Medical Journal found daily, or weekly, vitamin D supplements halved the risk of respiratory infections in people with the lowest levels of the vitamin. In people who had higher vitamin D levels, supplements cut their risk of an infection by 10 per cent.
Vitamin C may also be important for respiratory health. One large-scale study found it provides protection against COPD. Vitamin C's antioxidant properties have also been found to help the body fight off colds quicker. A 2013 review of studies concluded regular doses of 200mg shortened the duration of a cold (by eight per cent in adults and 14 per cent in children.)
Probiotics have also been shown to be beneficial for respiratory diseases. A 2017 review found they may improve respiratory and gastrointestinal conditions in those with cystic fibrosis.