The indoor environment in plain language — what each measurement means and why it matters.
Formally, particles with an aerodynamic diameter equal to or less than 2.5 micrometres, including inorganic ions, metal oxides and carbonaceous material. The smaller the particle, the deeper it travels into the respiratory tract.
Organic compounds with a vapour pressure of at least 0.01 kPa at 20 °C, or a boiling point of 250 °C or lower. In plain terms: gases emitted into the air from products or processes. They may be harmful on their own or form other pollutants through reactions in the air.
A naturally occurring gas produced by respiration and the combustion of fossil fuels. Indoors it is the most reliable proxy for ventilation: rising CO₂ means the air is being breathed more times before it is replaced.
A toxic gas produced by incomplete combustion — distinct from CO₂. It interferes with oxygen transport in the blood, which is what makes it dangerous at concentrations that are otherwise imperceptible.
Water vapour expressed as a percentage of the maximum the air can hold at a given temperature. It is driven by ventilation, moisture sources and temperature — and it is the variable that decides whether mould has a chance.
A fungus that thrives in damp environments and poses health risks. Prevention is not about cleaning; it is about keeping indoor humidity low enough that growth never starts.
Three bonded oxygen atoms. It causes respiratory issues and irritation of the eyes and throat, and it reacts with indoor pollutants to create further harmful compounds, formaldehyde among them.
A carcinogenic gas found in building materials, furniture and adhesives — plywood, particleboard and composite products in particular. A respiratory irritant that off-gasses over long periods.
A volatile hydrocarbon that enters homes through cigarette smoke, vehicle exhaust and household products. A known carcinogen, which makes awareness of exposure worthwhile.