Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless gas formed primarily as a by‐product of incomplete combustion. The major health hazard posed by CO is its capacity to bind with hemoglobin in the blood stream and thereby reduce the oxygen‐carrying ability of the blood. Transportation sources account for the bulk (76.6%) of total national CO emissions. Ambient CO concentrations have decreased significantly in the past two decades, primarily due to improved control technologies for vehicles. Areas with high‐traffic congestion generally will have high ambient CO concentrations: 80 urban areas in the United States are non‐attainment for CO. High localized and indoor CO levels can come from cigarettes (second‐hand smoke), wood‐burning fireplaces, and kerosene space heaters. Example 5.8 considers exposure to CO in the workplace.

EXAMPLE 5.8 TOTAL WEIGHTED AVERAGE CONCENTRATION

1. In a plant, employees were exposed to ambient CO throughout the day at different levels as follows:HourConc. (ppm)8–9 a.m.09–11 : 30 a.m.22511 : 30–12 : 30 p.m.10012 : 30–2 : 00 p.m.1652 : 00–4 : 00 p.m.45What is the TWA in ppm? If national ambient air quality standard is 50 ppm for 8‐hour, did employees’ exposure levels exceed the standard?
2. CO concentration in a working environment is 150 ppm. Working area is 1 150 000 ft³. How much air is needed to ventilate in the room to reduce the CO concentration to 50 ppm?
3. For safety reasons, ventilation dilution is not recommended. Why not? (Note that the molecular weight of CO is similar to that of air.)

SOLUTIONS

1. Eight‐hour weight averageTherefore, 125 ppm 8‐hour TWA concentration exceeded the national ambient air quality standard 50 ppm.
2. Assure steady state with perfect mixing and exponential decay.C = C_o × exp 50 = 150 exp ∴ t_a = 0.91 hours = 54.6 minutes∴ volume of air need to ventilate =  =  = 27 472 cfm
3. Incomplete combustion in the heater unit may increase the concentration of CO and add other noxious contaminants to the air in the warehouse. It is true, as well, that fixed dilution ventilation rate may not be adequate for CO removal during peak generation period. In an isolated warehouse environment, moreover, dilution ventilation may not adequately remove pockets of air that are high in CO concentration. In any event, CO has an acute toxicity, and dilute ventilation is not recommended for acutely toxic gases.

Lead and Mercury

Lead in the atmosphere is primarily found in fine particulates, up to 10 μm in diameter, which can remain suspended in the atmosphere for significant periods of time. Tetraethyl lead was used as an octane booster and antiknock compound for many years before its full toxicological effects were understood. The Clean Air Act of 1970 banned all lead additives and the dramatic decline in lead concentrations and emissions has been one of the most important yet unheralded environmental improvements of the past 25 years. In 1997, industrial processes accounted for 74.2% of remaining lead emissions, with 13.3% resulting from transportation, and 12.6% from non‐transportation fuel combustion (USEPA 1997).

Lead also enters waterways in urban runoff and industrial effluents and adheres to sediment particles in the receiving water body. Uptake by aquatic species can result in malformations, death, and aquatic ecosystem instability. There is a further concern that increased levels of lead can occur locally due to acid precipitation that increases lead’s solubility in water and thus its bioavailability. Lead persists in the environment and is accumulated by aquatic organisms.

Lead enters the body by inhalation and ingestion of food (contaminated fish), drinking water, soil, paint, and airborne dust. It is a corrosion by‐product from high‐lead solder joints in copper piping, old lead‐pipe goosenecks connecting to service lines to the water main, and old brass fixtures. The primary human health effect of lead in the environment is its effect on organs, tissues, and the brain development, especially in children. There is a direct correlation between elevated levels of lead in the blood and decreased IQ, especially in the urban areas of developing countries that have yet to ban lead as a gasoline additive.

According to EPA, coal‐fired power generation companies currently emit over 50 T of mercury every year in the United States. There was no quibbling that these levels were high and a potential health concern to humans and wildlife. Eating mercury‐tainted fish can trigger a variety of problems, ranging from hair loss and chronic fatigue in adults to nervous system impairment of fetuses and children.

Mercury taints the atmosphere worldwide, but there are large variations in how much of it drops onto land or water at any location. Recent experiments have begun identifying oxidizing gases, such as ozone and molecules containing the halogens bromine and chlorine, as triggers for that mercury fallout. Which oxidants dominate that process appears to depend on the environment, the season, the altitude of the airborne mercury, and even the amount of daylight.

Mercury enters the air easily. It is released when coal is burned, gold is mined, some chlorine is manufactured, and even when a fluorescent lightbulb breaks. Some 99% of the airborne metal is elemental. Fairly insoluble and unreactive in this form, it can circumnavigate the globe for up to two years. What is contaminating the Everglades, therefore may have originated in Miami, India, or Siberia.

However, atmospheric chemists have discovered that when elemental mercury encounters certain oxidants, it changes into so‐called reactive gaseous mercury. Unlike the element, this form is both highly reactive and water soluble, so it remains airborne only hours to days and falls – in rain or snow or attached to dust – near where it is formed. In a lake or ocean, bacteria transform it into methylmercury, the harmful form of the metal that fish and, in turn, people and other predators accumulate in their tissues. Symptoms of methylmercury poisoning include mental disturbance and impairment of speech, hearing, vision, and movement.

EXAMPLE 5.9 MERCURY REMOVAL

Medical sludge containing mercury is burned in an incinerator. The mercury feed rate is 0.92 lb/h. The resulting 500 °F product (40 000 lb/h of gas; MW = 32) is quenched with water to a temperature of 150 °F. The resulting stream is filtered to remove all particulates. What happens to the mercury? Assume the process pressure is 14.7 psi and that the vapor pressure of Hg at 150 °F is 0.005 psi.

SOLUTION

For the mercury to be removed by the filter, it must condense and form particulates. Therefore, the question to be answered relates to the partial pressure of mercury during removal compared to its vapor pressure at 150 °F.

• Molar flow rate of Hg = (9.3 lb/h)/(200.6 lb Hg/lbmol) = 0.046 lbmol/h
• Molar flow rate of gas = (40 000 lb gas)/(32 lb gas/lbmol) = 1250 lbmol/h
• Partial pressure = y_iP = 3.68 × 10⁻⁵(14.7 psi) = 5.41 × 10⁻⁴ psi.

Since the partial pressure is much less than the vapor pressure of 5 × 10⁻³ psi, mercury will NOT condense and thus will NOT be removed by the filter.

EXAMPLE 5.10 INHALABLE PCBS AND PAHS

Several industrial facilities emit inhalable pollutants including ethylene oxide (EO), poly‐chlorobiphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). The worker’s annual average exposure concentration of EO, PCBs, and PAHs are 10, 2, and 5 μg/m³, respectively.

Calculate the cancer risk caused by each pollutant and total cancer risk. Express results in additional cancer per million people. Use the following data given in a tabular form to solve this problem, along with the Eq. (5.5).

(5.5)

Unit risk data
Pollutant	Unit risk (m3/μg)
EO	8.8 × 10−5
PCBs	1.4 × 10−3
PAHs	1.7 × 10−3

SOLUTION

The cancer risk caused by ethylene oxide is established using the Eq. (5.5), and data presented in the problem statement:

The cancer risk by PCBs is estimated to be

The cancer risk by PAHs is estimated to be

The total cancer risk by inhalation is calculated from the arithmetic sum of individual cancer risks calculated above, assuming no interaction of pollutants in terms of carcinogenic effects.

Total cancer risk = 880 + 2 800 + 8 500 = 12 180 excess cancer cases per million people.

This rate is extremely high, 12 180 times higher than one in million cancer risk normally used as a basis for management of air toxics. This situation should be rectified as soon as possible by a significant reduction in air toxics releases. Also note that PAHs contributes the majority of risk to the workers at this facility.