Particulate

Particulates, alternately referred to as Particulate Matter (PM), aerosols or fine particles are tiny particles of solid or liquid suspended in the air. They range in size from 10 nanometres to 100 micrometres in diameter.

There are both natural and human sources of particulates. The biggest natural sources are wind-blown dust, volcanoes, and forest fires. Sea spray is also a large source of particles though most of these fall back to the ocean close to where they were emitted. The biggest human sources of particles are combustion sources, mainly the burning of fossil fuel in internal combustion engines in automobiles and power plants, and wind blown dust from construction sites and other land areas where the water or vegetation has been removed. Some of these particles are emitted directly to the atmosphere (primary emissions) and some are emitted as gases and form particles in the atmosphere (secondary emissions).

The composition of fine particles depends on the source. Wind-blown dust tends to be made of mineral salts and other crustal earth material. Primary emissions from combustion sources are made primarily of unburned fuel (hydrocarbons), elemental carbon (soot), elemental sulfur, mineral salts, and often contain traces of toxic metals. Secondary emissions are a combination of ammonia with either sulfuric acid or nitric acid and water.

In general, the smaller and lighter a particulate is, the longer it will stay in the air. Larger particles (greater than 10 micrometres in diameter) tend to settle to the ground by gravity in a matter of hours whereas the smallest particles (less than 1 micrometre) can stay in the atmosphere for weeks and are mostly removed by precipitation.

The size of the particle also determines where in the body the particle may come to rest if inhaled. Particles deposited in the airways and in the tracheobronchial region are removed rather efficiently by mechanical action, through the mucuous membrane escalator. Particles deposited in the alveolar region are removed by macrophages. There is evidence that particles smaller than 100 nanometres can pass through cell membranes. For example, particles can migrate into the brain. It has been suggested that particulate matter can cause similar brain damage as that found in Alzheimer patients.

The deposition site is a function of particle size. Larger particles are generally filtered by small hairs in the nose and throat and do not cause problems, but particulate matter smaller than about 10 micrometres, referred to as PM₁₀, can settle in the lungs and cause health problems. Particles smaller than 2.5 micrometres, PM_2.5, can penetrate deep into the lung and tend to be the most hazardous when inhaled. Particles emitted from modern diesel engines are typically in the size range of 100 nanometres, and thus are deposited preferentially in the alveolar region. In view of these deposition mechanisms, it is becoming increasingly clear that the legislative limits for engines, which are in terms of emitted mass, are not a proper measure of the health hazard. One particle of 10 µm diameter has approximately the same mass as 1 million particles of 100 nm diameter, but it is clearly much less hazardous, as it probably never enters the human body - and if it does, it is removed the next time you blow your nose. Proposals for new regulations exist in some countries, with suggestions to limit the particle surface area or the particle number.

The health effects of inhaling particulate matter has been widely studied in humans and animals and include asthma, lung cancer, and premature death. Particulate matter pollution is estimated to cause thousands of deaths per year in the United States (200 000 deaths per year in Europe). For this reason, the US Environmental Protection Agency (EPA) sets standards for PM₁₀ and PM_2.5 concentrations in urban air. EPA regulates primary particulate emissions and of precursors to secondary emissions (NOx, sulfur, and ammonia). Many urban areas in the US and Europe still frequently violate the particulate standards, though urban air has gotten cleaner, on average, with respect to particulates over the last quarter of the 20th century.

In directives 1999/30/EU and 96/62/EU, the European Union has set limits for PM₁₀ in the air:

	Phase 1 from 1 January 2005	Phase 2¹ from 1 January 2010
Yearly average	40 µg/m³	20 µg/m³
Daily average (24-hour) allowed number of exceedences per year.	50 µg/m³ 35	50 µg/m³ 7

¹ indicative value.de:Feinstaub nl:Fijn stof