The air we breathe is a heterogeneous mixture, a composite of gases, airborne solids, and liquids. Aerosols (mixtures of liquids and gases, or liquids and various chemical compounds including solids) and particulate matter are present in the air in concentrations that are variable over time and space. This report focuses on airborne natural materials derived from earth sources—the natural mineral, gaseous, and biological constituents of the earth’s surface that occur in the atmosphere and can cause either beneficial or adverse effects to human health and welfare.

Natural contaminants, such as wind-blown dust from arid areas, can carry bacteria and fungi. Such complexes of inorganic and biological particulate matter can travel within the troposphere for long distances (see Figure 3.1), even around the globe, in relatively short times. Other intermittent natural sources of harmful aerosols with obvious and immediate health impacts are emissions from volcanoes, including particulate matter as well as volcanogenic gases such as sulfur dioxide (SO₂) and carbon dioxide (CO₂). Active volcanoes, with their associated vents and fumaroles, have a long record of affecting populations worldwide; for example, the Icelandic eruption of Laki in 1783–1784 (see Box 3.1) caused many deaths in Europe, particularly of the infirm and the young (Grattan et al., 2005).

This chapter focuses on both direct health effects, such as the inhalation of suspended particulate matter (rock and soil particles) or gases (volcanic and biogenic gases, radon) that pose a health benefit or risk, and indirect effects, such as the inhalation of bacteria attached to soil particles.