One of the consequences of the current state of industrialization and an increasing demand for modern conveniences and improved quality of life has been an increased exposure to air pollutants from industrial activities, traffic, and energy production. Regulatory bodies, such as federal, state, and local environmental protection agencies, are responsible for assuring the public that the air is safe to breathe. These agencies are required to set standards, levels, and/or goals that will protect public health with an adequate margin of safety. These standards are established not only to protect healthy individuals, but also to protect sensitive population subgroups, such as children, asthmatics, the elderly, and individuals with emphysema, chronic obstructive pulmonary disease, or other conditions that render the group particularly vulnerable to air pollution. Although there is only one metal National Ambient Air Quality Standard (NAAQS) for lead, there are numerous other workplace and community-based screening levels, exposure limits, and reference concentrations for airborne metals that can be used as guidelines to set acceptable and appropriate levels of exposure and concern.
Assessing risk for metals in ambient air is difficult for a variety of reasons. Because organisms have always been exposed to metals, unlike synthetic organic substances, organisms have developed various means of responding to metals. There are major differences between the persistence of metals or inorganic metal compounds in the body and the persistence of organic compounds. Metals are neither created nor destroyed by biological and chemical processes, but may be biotransformed from one chemical species to another. That is, the metal ion thought to be responsible for the toxicity of a metal may persist in the body regardless of how the metal is metabolized. Some metals are considered essential for normal metabolic function, which is one of the primary factors that differentiate risk assessment for metals and metal compounds from that of synthetic organic chemicals.
Exposure to metals in the air is capable of causing a myriad of human health effects, ranging from cardiovascular and pulmonary inflammation to cancer and damage of vital organs. Contemporary research into air pollution is revealing that the metals components of particulate matter (PM) are contributing significantly to adverse health effects, even at the low concentrations found in ambient air. The EPA set health-based standards for fine particulates in 1997, but the standards do not take into account new research on the composition of the particulate matter or the toxicity of its components. The toxicity of particulate matter, in particular the fine (1 to 2.5 microns [μm]) and ultrafine particles (0.1 to 1 μm), has been proven to cause severe mortality and morbidity in humans over the past 25 years; however, in the past decade, emerging research is providing evidence that the metallic particles may be more dangerous than other PM components. In fact, current evidence is showing that mass concentration of PM alone may not be the best indices for associating health effects with exposure to PM.
The aerodynamic size and composition of particles determine their behavior in the mammalian respiratory system. Particle size is one of the most important parameters in determining the atmospheric lifetime of particles, which may be a key consideration in assessing inhalation exposures, as well as exposures related to exposure pathways involving deposition onto soil or water. Metals emitted by combustion processes (e.g., the burning of fossil fuels or wastes) generally occur in small particles or the fine fraction, which is often characterized by particles less than 2.5 μm in diameter (PM2.5). In contrast, the larger sized, course mode particles result from mechanical disruption, such as crushing, grinding, evaporation of sprays, or suspensions of dust from construction and agricultural operations. Accordingly, metals in course mode particles (i.e., those larger than approximately 1–3 μm) are primarily those of crustal origin, such as aluminum, zinc, and iron.
Generally, the evaluation of most studies shows that the smaller the size and greater the solubility of the PM, the higher the toxicity through mechanisms of oxidative stress and inflammation. A study of PM2.5 in 2010 showed that metals were the important source for cellular oxidant generation and subsequent health effects. Health effects are stronger for fine and ultrafine particles for a variety of reasons:
- The studies of the size distribution of metals show that most of the toxic metals accumulate in the smallest particles (PM2.5 or less).
- This size fraction can penetrate deeper into the airways of the respiratory tract and predominantly deposits in the alveolar region of the lungs, where the adsorption efficiency for trace elements varies from 60–80%.
- A fine metallic particle in contact with lung tissue/cells involves the release of metal ions into the biological system.
- Ultrafine particles are known to have increased solubility, as compared to larger size particles of the same composition because of the increased surface-to-volume ratio for smaller particle sizes.
- Fine and ultrafine particulate matter have the longest residence time in the atmosphere (~100 days), which allows for a large geographic distribution.
- Recent studies have shown that the metals component in fine and ultrafine PM is particularly toxic and are the primary contributors to negative human health.
- Furthermore, these particles also play a significant role in global climate change and can be transported over long distances by prevailing winds.
These consequences require us to give priority to the chemical characterization of the fine and ultrafine fraction of airborne particles to understand their possible implication to health effects.
In conclusion, for the effective management of air quality, great importance must be attached to the identification of both the sources and characterization of suspended PM. Source apportionment provides an estimate on the PM contribution of various sources to the levels at the receptor; it is also a key component necessary for developing and achieving desired air-quality objectives. The results of source apportionment can be used to evaluate emissions reduction on the PM levels and to devise more efficient emission reduction strategies. Therefore, estimating the airborne PM mass concentration, as well as individual chemical/metal speciation, is critical not only for comparing with recommended values, but also to identify the major sources that affect a particular area. This knowledge will also help regulators both foresee and prevent threats and risks before they become problems.