Health Effects of Diesel Particulates

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Abstract: The health effects of diesel particulates, a complex mixture of solids and liquids, are not yet well understood. Biological activity of particulate matter may be related to particle sizes and/or particle composition. A number of epidemiological studies concluded that exposure to particulate matter may cause increased morbidity and mortality, such as from cardiovascular disease. Long-term exposure to diesel exhaust is also associated with small increase in the relative risk of lung cancer.

Introduction

Components of Diesel Particulates

Diesel particulate matter (PM, also abbreviated DPM) is a complex mixture of solid and liquid material. The particles in diesel exhaust are of special concern because, due to their respirable size, they can penetrate deep into human lungs. The composition of DPM includes many species that are known for their adverse health effects, including several carcinogens. Health effects most often linked with particulate pollution, from diesel and other sources, include an increase in death due to respiratory and cardiovascular disease and worsening of symptoms in people with asthma.

Primary materials found in the solid phase of diesel PM include inorganic carbon and metal ashes, while the liquids include high boiling hydrocarbons, water, and sulfuric acid. According to most legal definitions of diesel particulates and the corresponding sampling and measurement methods, diesel particulates are sampled from exhaust gases that are diluted with air and cooled, typically to 52°C. These sampling procedures have been introduced in an attempt to simulate atmospheric dilution effects on diesel emissions.

Although it is known today that the products of real life atmospheric dilutions may differ from what is sampled in a laboratory dilution tunnel, there has been a consensus in regards to the above definition among worldwide public health authorities. Accordingly, the total particulate matter traditionally includes the following fractions:

  1. Solids (inorganic carbon, ash)
  2. Organics (soluble organic fraction—SOF)
  3. Sulfates (hydrated sulfuric acid, metal sulfates).

There is no such consensus, however, in diesel particulate exposure regulations in various occupational health environments. The elemental carbon (EC) definition—which excludes all organic content, sulfates, or inorganic ash—appears to become the preferred representation. Sometimes, the total carbon (TC) definition, which includes both the elemental and organic carbon but no sulfates, is also used.

Once released into the atmosphere, diesel PM becomes a part of the ambient suspended particulate matter, which also includes a number of other components, such as:

Ambient particulates are measured according to a large variety of protocols, but in general they do include both solid and liquid material, just as it was the case with diesel PM.

Health effects from exposure to diesel PM and non-diesel particles are extremely difficult to separate. Since particulates from different sources are often similar, so are the health effects they may cause. In fact, health effects caused by some gaseous pollutants may be also similar to those caused by particles. Despite these challenges, research studies have always attempted to separate the toxic effects of gaseous and particle pollution from different sources. This type of knowledge is essential for the policy makers to establish control strategies that would maximize the public health benefit by targeting the most critical sources of pollution.

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