Assessing the Risks of Some Common Pollutants

Risk assessment is built on the principle that small exposures carry with them some risk of an untoward health effect such as development of a malignant tumor or leukemia at some time in the future. Such risks are generally considered to be stochastic or probabilistic in nature and are expressed in terms of a risk coefficient, an expression of the probability or chance of the specific health effect occurring per unit of exposure. Although response to ionizing radiation is generally considered to be linear with dose, the risk from chemical carcinogens may not be; thus, risk evaluation needs to consider such effects as the existence of a threshold dose below which the effect does not occur, as well as the shape of the dose–response curve.

Also, the latency period, the time between the exposure and the onset of disease, needs to be taken into account. For example, if the latency period is 25 years and exposure occurs at age 50, the individual may die from other causes prior to developing the specific health effect under consideration. Even if the individual lives long enough for the health effect to be manifested, the number of years of life lost, and productive years of life lost, will be less than if the exposure had occurred at an earlier age. Health and safety risks are sometimes expressed in terms of the number of years of productive (or health) life lost per thousand worker‐years, as shown in Example 5.7.

EXAMPLE 5.7 CANCER RISK ASSESSMENT

Estimate the total number of fatal leukemias in 50 years to a population of 100 000 exposed to an average annual dose of 500 μGy if the risk coefficient is 0.03 Gy⁻¹.

SOLUTION

NO_x, Hydrocarbons, and VOCs: Ground‐Level Ozone

Ground‐level ozone is one of the most pervasive and intractable air pollution problems in the United States. In more than 20 urban areas, its concentration exceeds one or more of the ambient air quality standards. In the language of regulators, these areas are said to “non‐attainment.” (We emphasize difference between the “bad” ozone created at or near ground level from the “good” or stratospheric ozone that protects us from UV radiation.)

Ground‐level ozone, a component of photochemical smog, is actually a secondary pollutant in that certain precursor contaminants are required to create it. The precursor contaminants are nitrogen oxides (NO_x, primarily NO and NO₂) and hydrocarbons. The oxides of nitrogen along with sunlight cause ozone formation, but the role of hydrocarbons is to accelerate and enhance the accumulation of ozone.

Smog Formation

The most important process for ozone formation in the lower atmosphere is photodissociation of NO₂:

Where M is molecular oxygen or nitrogen oxide. This cycle results in O₃ concentration being in a photostationary state dictated by the NO₂ photolysis rate and ratio of [NO₂]/[NO]. The role of VOCs in smog formation is to form radicals which convert NO to NO₂ without causing O₃ destruction, thereby increasing the ration [NO₂]/[NO], and increasing O₃.

The tendency of individual VOCs to influence O₃ levels depends upon its hydroxyl radical (▪OH) rate constant and elements of its reaction mechanism, including radical initiation, radical termination, and reaction which removes NO_x. Simplified smog formation potential indices have been proposed based only on VOC hydroxyl radical rate constants.

Oxides of nitrogen (NO_x) are formed in high‐temperature industrial and transportation combustion processes. In 2016, US transportation sources accounted for 56.6% and non‐transportation fuel combustion contributed 34.4% of total NO_x emissions. Health effects associated with short‐term exposure to NO₂ (<3 hours at high concentrations) are increased respiratory illness in children and impaired respiratory function in individuals with preexisting respiratory problems.

Major sources of hydrocarbon emissions are the chemical and oil‐refining industries, and motor vehicles. In 2016, industrial processes accounted for 80.5%, while the transportation sector contributed 19.5% of the total of manmade (non‐biogenic) hydrocarbon sources, while transportation sources accounted for 18.1% and non‐transportation sources for 81.9% of total VOCs (both anthropogenic and biogenic sources), respectively. Solvents comprise 66% of the industrial emissions and 34% of total VOC emissions. It should be noted that there are natural (biogenic) sources of HCs/VOCs, such as isoprene and monoterpenes that can contribute significantly to regional hydrocarbon emissions and low‐level ozone levels.

Also, ground‐level ozone concentrations are exacerbated by certain physical and atmospheric factors. High‐intensity solar radiation, low‐prevailing wind speed (dilution), atmospheric inversions, and proximity to mountain ranges or coastlines (stagnant air masses) all contribute to photochemical smog formation. Figure 5.2 shows a picture of downtown Los Angeles sky covered with smog cause poor visibility (New York Times, 17 February 2018b).

Human exposure to ozone can result in both acute (short‐term) and chronic (long‐term) health effects. The high reactivity of ozone makes it a strong lung irritant, even at low concentrations. Formaldehyde, peroxyacetylnitrate (PAN), and other smog‐related oxygenated organics are eye irritants. Ground‐level ozone also affects crops and vegetation adversely when it enters the stomata of leaves and destroys chlorophyll, thus disrupting photosynthesis. Since ozone is an oxidant, materials with which, such as rubber and latex painted surfaces, undergo deteriorations.

The perfumes, deodorants, soap that keep us smelling good are fouling the air with a harmful type of pollution – at levels as high emissions from today’s cars and trucks. Researchers found that petroleum‐based chemicals used in perfumes, paints, and other consumer products can, taken together, emit as much air pollution in the form of VOCs, as motor vehicles do. The VOCs interact with other particles in the air to create the building blocks of smog, namely ozone, which can trigger asthma and permanently scar the lungs, and another type of pollution known as PM_2.5, fine particles that are linked to heart attacks, strokes, and lung cancer. Smog is generally associated with cars, but since the 1970s regulators pushed automakers to invest in technologies that have substantially reduced VOC emissions from automobiles. California Air Resources Board’s consumer products and air quality’s preliminary results also suggested that emissions, both point and fugitive sources, from those products and processes were higher than previously estimated (New York Times, 17 February 2018b).

There are tens of thousands of chemicals in consumer products and researchers have not yet pin‐pointed which chemicals are most likely to form ozone or PM_2.5 particles. We need to remind ourselves that our energy sources and consumer products we use every day are continuously changing the composition of our atmosphere.