Issue link: https://hi.iaq.net/i/670243
Every breath we take: the lifelong impact of air pollution Ozone (O 3 ) This is a gas consisting of three oxygen atoms joined together. It is formed by chemical reactions between other air pollutants, in particular the reaction of oxides of nitrogen with carbon compounds called volatile organic compounds (VOCs) in the presence of sunlight. These reactions take place over periods of several hours or even days. Ozone levels therefore tend to be higher in the countryside than in cities, and greater in summer than winter. Some ozone travels over large distances. Ozone levels fluctuate markedly over time and are highest in hot, bright weather. Particulate matter (PM) Particles in the air are a complex mix of materials. The health effects covered by this report mostly relate to particles that are generated by human activity ('anthropogenic' particles), either directly or by chemical reactions in the air. However, some natural dusts (eg soil and sand particles) may also be small enough to be breathed into the lungs. Particle concentrations are usually expressed as mass (micrograms; g) per cubic metre of air (m 3 ). Historically, concentrations of particulate matter (PM) were determined optically by measuring the 'blackness' of a filter after air had been drawn through it. This measurement was called black smoke and is practically the same as the black carbon measurement, as this is the part of PM that most strongly absorbs light. Black carbon is created by the incomplete combustion of fuels and is a major part of soot – for example diesel soot. Besides elemental carbon it contains polycyclic aromatic hydrocarbons (PAHs). The 'black smoke' metric has been largely replaced by metrics of PM based on the aerodynamic diameter of the particles. Thus PM 10 is the amount of particulate matter that is generally less than 10 µm in diameter. PM 2.5 is the smaller fraction of PM 10 and consists of particles <2.5 µm across (often referred to as 'fine' particles). Coarse PM is the difference between PM 10 and PM 2.5 . Ultra-fine particles are the smallest fractions of PM 2.5 and are <0.1 µm (100 nm) in diameter. Differentiation of PM into fractions of different sizes is very relevant for health studies because fractions of <10 millionths of a metre (10 µm) in diameter tend not to be filtered out by the nose, and are able to penetrate down into the lungs. In contrast, very small particles (nanoparticles), which range in size from 0.1 to 0.001 µm, are so small that they can pass into the circulation. The surface area:volume ratio of particles increases exponentially as particles become smaller and smaller. PM consists of carbon, ammonium nitrate and ammonium sulphate, oxides and salts of many metals, and organic materials including rather large and complex molecules and fragments of soil. The composition of the PM mixture varies from place to place. Near to roads, emissions from vehicles form the majority of PM, whereas in rural areas there is xviii © Royal College of Physicians 2016