Air Pollution's Most Abundant Metal Villain

Air pollution is a pressing issue that has been estimated to cause 6.7 million premature deaths worldwide annually. Fine particulate matter (PM2.5) is believed to be the largest contributor to the adverse health effects caused by air pollution. These fine particles can be derived from primary sources, such as the combustion of fuels in power generation facilities, industries, or vehicles, and secondary sources, such as chemical reactions between gases. While the specific toxic constituents of PM2.5 have not been definitively determined, this article aims to explore the potential health risks associated with the inhalation of particulate metals, commonly found in air pollution, and their sources.

Particulate matter (PM) from combustion of polluting fuels

Particulate matter (PM) refers to inhalable particles composed of sulphate, nitrates, ammonia, sodium chloride, black carbon, mineral dust, water, and other compounds. PM is a mixture of solid particles and liquid droplets found in the air. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small that they can only be detected using an electron microscope.

PM emissions are the result of the formation of soot or black carbon, which are frequently referred to as fine or ultrafine particles in the environment. Both solid and liquid particles are included in this category, with a diverse composition that includes carbonaceous particles, nitrates, and sulfates, among other compounds. The PM emissions can be categorized into volatile particulate matter (vPM) and nonvolatile particulate matter (nvPM).

The combustion of polluting fuels in open hearths or poorly vented, inefficient stoves, or space heaters is the greatest source of particulate matter in households. Household activities such as cooking, space heating, and lighting with kerosene also contribute to indoor air pollution. In outdoor environments, the main sources of PM are typically traffic and transportation, industrial activities, power plants, construction sites, waste burning, fires, and fields.

The health risks associated with particulate matter, especially PM10 and PM2.5, are well documented. PM can penetrate deep into the lungs and enter the bloodstream, causing cardiovascular and cerebrovascular issues, as well as respiratory problems. PM2.5, or fine particles, are the main cause of reduced visibility (haze) in parts of the United States and other countries.

The formation of particulate matter is due to the incomplete combustion of polluting fuels and depends on factors like engine load, combustion chamber geometry, O2 concentration, fuel composition, and properties. Oxygenated fuel has the potential to reduce exhaust particle size and promote complete combustion, thereby reducing particulate matter emissions. The presence of organometallics in fuels, as additives, leads to the formation of inorganic metal oxides on flame oxidation, thus forming PM.

Health risks of heavy metals from air pollution

Heavy metals are well-known environmental pollutants due to their toxicity, ability to remain in the atmosphere, and bioaccumulation in the human body. They are characterised by their high atomic mass and toxicity to living organisms. Most heavy metals occur naturally, but some are anthropogenic. Heavy metals can become more toxic when they mix with other environmental elements like water, soil, and air. Humans and other living organisms can be exposed to these metals through the food chain.

The heavy metals cadmium, lead, and mercury are common air pollutants, largely due to industrial activity. Even at low levels in the atmosphere, these metals contribute to a buildup in the soil, where they persist and accumulate in the food chain, both on land and in water. Heavy metals have been associated with a wide range of health conditions, including kidney and bone damage, developmental and neurobehavioral disorders, elevated blood pressure, and potentially even lung cancer.

Particulate matter (PM) refers to inhalable particles composed of sulphate, nitrates, ammonia, sodium chloride, black carbon, mineral dust, or water. PM can vary in size, with the largest particles (coarse particles) having a diameter between 2.5 µm and 10 µm, and the smallest particles (finer particles) being less than 2.5 µm in diameter. Coarse particles are primarily composed of pollen, sea spray, and wind-blown dust from erosion, agriculture, roadways, and mining operations. Finer particles can be derived from primary sources, such as the combustion of fuels in power generation facilities, industries, or vehicles, and secondary sources, such as chemical reactions between gases.

The health risks associated with PM, particularly PM10 and PM2.5, are well documented. These particles can penetrate deep into the lungs and enter the bloodstream, leading to cardiovascular issues (such as ischaemic heart disease), cerebrovascular problems (like strokes), and respiratory illnesses.

To address the health risks posed by heavy metals and particulate matter in air pollution, various treatment options are being explored. Nanotechnology, for instance, offers promising possibilities for the analysis and removal of heavy metals from complex matrices using nanomaterials such as graphene, magnetic nanoparticles, and carbon nanotubes.

Nitrogen dioxide from vehicle exhaust

Nitrogen dioxide (NO2) is a major gaseous pollutant that is primarily released from combustion sources such as vehicles, factories, and power plants. Vehicle exhaust is a significant source of nitrogen dioxide, which is a harmful component of air pollution.

NO2 is formed during the combustion of diesel fuel in vehicle engines, particularly in diesel vehicles equipped with emission control technologies such as Diesel Oxidation Catalysts. The switch from petrol to diesel vehicles has exacerbated the problem, as diesel combustion produces higher levels of NOx, a mixture of nitric oxide (NO) and NO2. In the case of diesel fuel, up to 70% of the NOx produced is NO2, compared to only 10-15% when petrol is burned.

NO2 emissions from vehicle exhaust have been a significant contributor to near-road ambient NO2 concentrations, particularly in European cities. However, recent data suggests that absolute NOx and NO2 emissions from newer diesel cars and light commercial vehicles have decreased substantially since around 2007. Additionally, it has been found that NO2 emissions decrease as vehicle mileage increases.

Nitrogen dioxide has direct and indirect impacts on human health and the environment. NO2 reacts with atmospheric oxygen to form tropospheric ozone, which is harmful to plant and animal cells. It also contributes to the production of fine particulate matter (PM2.5), which can penetrate deep into the lungs and enter the bloodstream, causing cardiovascular and respiratory issues. The health risks associated with PM2.5 are well documented, and exposure to this pollutant can lead to both short-term and long-term health problems.

Reducing NO2 emissions from vehicle exhaust is crucial for improving air quality and protecting public health. This can be achieved through various measures, including the implementation of clean air zones, fuel duties, and incentives for transitioning to cleaner modes of transportation, such as electric or hybrid vehicles.

Carbon monoxide from vehicle exhaust

Carbon monoxide (CO) is a highly toxic, colourless, and odourless gas that is produced through the incomplete burning of fuels such as propane, coal, wood, oil, kerosene, natural gas, and gasoline. One of the most common sources of CO exposure is the internal combustion engine, a primary component of gasoline-fuelled automobiles. The typical internal combustion engine used in most cars and trucks can produce high concentrations of CO. Faulty or poorly maintained exhaust systems can result in increased CO levels within the cabin of the car or truck.

The combustion process in automobiles generates high amounts of carbon monoxide—over 30,000 parts per million (ppm) for the typical vehicle. To control exposure, cars are equipped with a catalytic converter, which combines oxygen with carbon monoxide to form carbon dioxide. Once the substance passes through the exhaust manifold, the amount of carbon monoxide is reduced to about 1,000 ppm. Without this process, a car can release 30,000 ppm or higher concentrations of carbon monoxide from the tailpipe.

A defective exhaust system can cause carbon monoxide to leak into the cabin of the vehicle. This may be due to a cracked exhaust manifold, damaged seals, or holes in the floor or side panel. Driving with the trunk lid or rear tailgate open can also increase the risk of carbon monoxide entering the vehicle. The catalytic converter is ineffective when there is insufficient oxygen in the area, which can occur in closed spaces like garages. As a result, it is dangerous to leave a car running in a garage, even if the garage door is open.

Carbon monoxide poisoning can occur within minutes of exposure. Operating a vehicle with a defective exhaust system or poorly tuned engine can increase the risk of poisoning. Exposure to carbon monoxide causes it to displace oxygen in the bloodstream, leading to oxygen deprivation in vital organs such as the heart, lungs, and brain. Symptoms of carbon monoxide poisoning include headache, fatigue, eye irritation, difficulty thinking or reasoning, slower reactions, and chest pain.

To prevent carbon monoxide poisoning, it is important to have the exhaust system inspected regularly for signs of rust and to check connections between parts. It is also recommended to avoid driving with the trunk lid or rear tailgate open and to avoid warming up a vehicle in a garage, even with the door open. Keeping a portable CO detector in the car can help alert individuals to dangerous levels of carbon monoxide.

Sulfur dioxide from factories burning fossil fuels

Sulfur dioxide (SO2) is a major air pollutant, particularly associated with the burning of fossil fuels in factories and power plants. SO2 is a harmful gas that is released into the atmosphere when fossil fuels, such as coal, oil, and natural gas, are burned for energy production. The burning of fossil fuels has been identified as the primary driver of climate change, significantly impacting the Earth's ecosystems and causing a range of health issues for humans and the environment.

Power plants and industrial facilities are the largest sources of SO2 emissions. In the process of generating electricity, these facilities burn fossil fuels, particularly coal, which contains sulfur as an impurity. The combustion of coal releases sulfur dioxide into the atmosphere, contributing to air pollution. The Midwest states in the US, including Illinois, Indiana, and Ohio, are notable for their high sulfur content in coal, supplying a significant portion of the country's coal needs.

The release of SO2 from factories burning fossil fuels has significant environmental and health consequences. Sulfur dioxide combines with other substances in the atmosphere, such as water vapour and oxygen, to form acid rain. Acid rain damages both natural and human-made environments, including ecosystems and buildings. It contributes to the corrosion and deterioration of structures, particularly those made of stone or metal. Additionally, acid rain harms aquatic ecosystems by increasing the acidity of lakes, rivers, and streams, which can be detrimental to fish and other aquatic life.

Moreover, SO2 emissions from factories contribute to poor air quality, posing risks to human health. Fine particles of sulfuric acid and sulfate particulate matter can travel long distances, affecting communities located hundreds of miles away from the source of pollution. These pollutants are associated with respiratory issues, triggering asthma attacks and causing other respiratory ailments. The inhalation of SO2 and associated particulate matter can also lead to cardiovascular problems, including heart attacks.

To address the issue of SO2 pollution, governments and regulatory bodies, such as the EPA in the United States, have implemented measures to reduce emissions and improve air quality. These include setting national air quality standards and identifying areas that exceed safe levels of SO2. Subsequently, local and tribal governments develop plans to mitigate and reduce SO2 levels in their respective regions. By controlling and reducing SO2 emissions, there is a co-benefit of decreasing other sulfur oxide (SOx) pollutants, improving air quality, and reducing the formation of particulate sulfur pollutants.

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