Particulate matter (PM) is a range of dust, dirt and liquids suspended in the air. Some particles like smoke, smog and soot can be seen, but the most harmful are smaller invisible particles which can get into your lungs and bloodstream

Carbon dioxide is an invisible gas that comes from human breathe that can cause headaches, restlessness and drowsiness. High levels are correlated to reduced cognitive function and the increased transmission of infectious diseases

Volatile organic compounds (VOCs) are odours and chemicals emitted by many everyday products like household cleaning materials, paints, furniture and plastic toys. They can cause eye, nose and throat infections and even liver and lung damage in serious cases

Nitrogen dioxide is produced by road traffic, power plants and other fossil fuel combustion processes causing chronic respiratory conditions leading to coughing, wheezing and difficulty breathing

Aerosol droplets exhaled when breathing, talking or singing can float in the air for more than 30 minutes, and can carry Covid-19 and other infectious viruses and bacteria which can then be breathed in by other people

Too much or too little humidity can affect allergies, asthma and cold/flu symptoms. When humidity is too high mould and damp will occur, and when too low, humidity causes dry skin and increased susceptibility to colds and respiratory illnesses

Mould spores are a microscopic fungus that are invisible to the human eye, but in damp, warm conditions can form new visible colonies that cause property damage and serious respiratory conditions

Hay fever is an allergic reaction to pollen, typically when it comes into contact with your mouth, nose, eyes and throat. It is caused by the immune system releasing histamine as well as other chemicals into the body to fight off the allergens