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Air Pollutants

Criteria Air Pollutants

Sun setting with a cloud-filled sky

The Clean Air Act regulates six common air pollutants: particle pollution (particulate matter), ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. These are called “criteria” air pollutants because the Environmental Protection Agency sets human health-based and environmentally-based criteria for setting limits on the amount of these pollutants that are permissible in the ambient air. These limits are called primary and secondary standards.

In 1997 particle pollution was divided into two divisions, Particulate Matter (PM) 10, particles equal to or smaller than 10 micrometers in diameter, and Particulate Matter (PM) 2.5, particles equal to or smaller than 2.5 micrometers in diameter. At one time the belief was that if you pulverized particles, the danger went away. Health care professionals learned that pulverizing particles actually increased the danger because microscopic particles get deep into the lungs.

Ozone and particle pollution are the most widespread health threats.

Hazardous Air Pollutants

U.S. EPA and Iowa DNR regulate 188 air pollutants known or suspected to cause cancer or other serious health effects such as reproductive effects or birth defects, or adverse environmental consequences. These pollutants are called hazardous air pollutants (HAP) or air toxics.

Air toxics are generally more localized than the criteria pollutants and the highest levels are close to their sources. Most air toxics originate from man-made sources, including cars and trucks, factories, power plants and refineries, as well as some building materials and cleaning solvents.

U.S. EPA is responsible for setting national standards for regulating sources of HAP, called National Emission Standards for Hazardous Air Pollutants (NESHAP).

Because of the difficulty in assessing air toxic health risks with the original Clean Air Act enacted in 1970, Congress amended the Clean Air Act in 1990 to emphasize controlling emissions of air toxics through available control technology, and then periodically evaluating any remaining risk from air toxics.

  Primary Standards Secondary Standards
Pollutant Level Averaging Time Level Averaging Time
Carbon Monoxide
9 ppm
(10 mg/m3)
8-hour None
35 ppm
(40 mg/m3)
0.15 µg/m3 Rolling 3-month Average Same as Primary
Nitrogen Dioxide
53 ppb Annual
(arithmetic average)
Same as Primary
100 ppb 1-hour None
Particulate Matter < 10 µm
150 µg/m3 24-hour Same as Primary
Particulate Matter < 2.5 µm
9.0 µg/m3* Annual
(arithmetic average)
15.0 µg/m3 Annual
(arithmetic average)
35 µg/m3 24-hour Same as Primary
0.070 ppm 8-hour Same as Primary
Sulfur Dioxide
75 ppb 1-hour 0.5 ppm 3-hour
Current as of May 10, 2024. Units of measure for the standards are parts per million (ppm) by volume, parts per billion (ppb - 1 part in 1,000,000,000) by volume, milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (µg/m3).
* The current annual PM2.5 NAAQS became effective for PSD applications on 5/6/2024. The DNR will continue to implement the previous annual PM2.5 NAAQS (12 µg/m 3) for non-PSD applications until our revised PM2.5 Infrastructure SIP is submitted to EPA (early 2027).

For more information about the NAAQS, go to

lead is a toxic, naturally occurring metal used in some manufactured products found in and around the home (such as paints made before 1978). the major source of lead emissions was once from the combustion of leaded gasoline. removing lead from gasoline reduced lead emissions by 95 percent between 1980 and 1999. today the sources of highest lead emissions are from lead smelters, waste incinerators, coal combustion, lead-acid battery manufacturers, and piston aircraft burning leaded aviation gasoline.

the epa health standard for lead emissions was recently lowered from 1.5 micrograms per cubic meter of air (μg/m3) to 0.15 (μg/m3) to increase protection for children and others at-risk for lead exposure. lead is persistent in the environment and accumulates in soils and sediments when lead emitted into the air settles to the ground or is captured in falling raindrops. lead which has previously accumulated on the ground or in the soil can be re-emitted if disturbed, for example, by a construction project.

airborne lead can be inhaled or ingested. ingestion is the main route of human exposure. once in the body, lead distributes throughout the blood circulation system and builds up in the bones. lead is associated with a broad range of health effects, including the oxygen-carrying capacity of the blood, central nervous system, cardiovascular system, kidneys and immune system.

children are more vulnerable than adults to the damaging effects of lead. they breathe 1.5 times as much air proportionately than adults. children typically spend more time outdoors and are more physically active. lead exposure also occurs through hand-to-mouth activities. because children’s brains are still developing, they are more susceptible to the poisonous effect.

federal regulations require lead ambient air monitors be placed at facilities that emit at least.0.5 tons per year and where urban areas have a population of 500,000 people or more beginning in 2012.

for more information about lead, go to

nitrogen oxides are emitted from high temperature combustion sources such as autos, trucks, aircraft and from boilers used to provide heat, steam or electricity.

since air is made up of almost 80 percent nitrogen, when high temperature burning occurs, some of the nitrogen in the air is burned to release nitric oxide or nitrogen dioxide gas. these gases can form a reddish-brown haze over urban areas or areas near large emitters.

airborne nitrates reduce visibility, contribute to acid rain, play a major role in the formation of ozone smog or react with other chemicals to form particulate matter. these particles can fall to earth in rain or snow to increase nitrogen levels in soils and water bodies. nitrates deposited into water contribute to algae blooms that can cause depleted oxygen. for example, a significant portion of nitrogen that enters the chesapeake bay comes not from surface runoff or water discharges, but from airborne nitrates. what percent of nitrates in iowa water originated from airborne deposition is currently unknown.

nitrogen oxides have gone unmonitored in iowa since the 1980s. the state renewed monitoring in 2000.

Image depicting the formation of ozone in the atmosphere (VOCs + NOx + Heat + Sunlight = Ozone)ozone is formed during a photochemical reaction, meaning several common airborne pollutants react with sunlight to form another pollutant called ozone. ideal conditions for ozone formation require warm, windless days with bright sunlight found during the summer and early fall.

during these conditions, volatile organic compounds (vocs) react with nitrogen oxides (nox), also called “ozone precursors” to form ozone. volatile organic fumes come from evaporation of gasoline, paint, solvents, consumer products, varnishes, and industry chemicals.

nitrogen oxides come from high-temperature combustion found in exhaust from auto and truck engines, boilers, utilities and other sources. the concentration of these precursor gases, the volume of air to dilute and mix, the temperature and intensity of ultraviolet light affect this process.

both urban and rural areas of the state are subject to elevated ozone levels as winds carry emissions hundreds of miles away from their original sources.

ozone occurs in two layers of the atmosphere. the layer closest to the earth’s surface is the troposphere. here, ground-level or “bad” ozone is an air pollutant that is harmful to breathe and it damages crops, trees and other vegetation.

six miles up is the second layer of atmosphere called the stratosphere. the stratosphere or “good” ozone layer extends upward from about 6 to 30 miles and protects life on earth from the sun’s harmful ultraviolet (uv) rays.

ozone loss in the stratosphere can cause increased amounts of uv radiation to reach the earth which can lead to increased cases of skin cancer, cataracts, and impaired immune systems. overexposure to uv is believed to contribute to the increase in melanoma, the most fatal of all skin cancers. since 1990, the risk of developing melanoma has more than doubled. uv can also damage sensitive crops, such as soybeans, and reduce crop yields.

current scientific evidence links short-term exposures to sulfur dioxide, ranging from 5 minutes to 24 hours, with an array of adverse respiratory effects including bronchoconstriction and increased asthma symptoms. these effects are particularly important for asthmatics at elevated ventilation rates (e.g., while exercising or playing). studies also show a connection between short-term exposure and increased visits to emergency departments and hospital admissions for respiratory illnesses, particularly in at-risk populations including children, the elderly, and asthmatics.

sulfur dioxide is the leading contributor to acid precipitation that can harm water bodies, fish and amphibian populations and forests across the upper midwest, northeast and canada. acid rain also impacts portions of the rocky mountains and other areas.

sulfuric gases and particles can slowly degrade building materials such as brick and mortar, pipes and metal surfaces, paints, stone and monuments. airborne sulfates, along with particulate and nitrogen oxides, also contribute to visibility loss or haze. in iowa, visibility loss due to sulfates and other airborne particles may be approximately one-third natural visibility on average days and less on the worst days. in other areas of the nation, such as national parks, air pollution has cut visibility in these scenic, "pristine" areas by 80 percent on the worst days. generally, visibility is worst in the eastern states and improves in western states, although visibility is degraded nationally.

when sulfur-containing fuels such as gasoline, coal and fuel oil are burned, the sulfur is released. citizens can help reduce emissions by conserving electricity, properly maintaining vehicles, driving less and by consolidating errands.


How Air Pollution is Controlled

Pollution control equipment can reduce emissions by cleaning exhaust and dirty air before it leaves the business. A wide variety of equipment can be used to clean dirty air. DNR engineers carefully study and review how these controls may work and the methods and requirements are put into a permit - a major duty performed by the DNR.

There are other ways to reduce emissions besides using pollution control equipment, such as preventing emissions to begin with. Air quality permits help minimize, reduce or prevent emissions as much as possible by placing requirements on how things are done.

Permits can specify the quantity, type, or quality of fuel or other substance used in a process. For example, a permit might specify the maximum percent of sulfur that can exist in the coal to reduce sulfur dioxide emissions. A permit may specify the quantity of volatile chemicals in paint, solvent, adhesive or other product used in large quantity during manufacturing. Permits can also help reduce the impact of emitted pollutants on local air by specifying smokestack height and other factors.

DNR Engineers can also set combustion specifications to minimize emissions. For example, to help reduce nitrogen oxide formation, the combustion conditions in the furnace can be altered. The flame temperature can be lowered or raised, the amount of time air remains in the combustion chamber can be altered, or the mixing rate of fuel and air can be changed. These options are often reviewed, studied and best choices made depending upon cost, plant design and many other variables.

Common control equipment and how it works is explained below:

Animated depiction of an adsorbant material

Adsorption is similar to using a sponge to soak up water. A porous solid material is used to soak up gaseous air contaminants from the exhaust air. Inside an adsorber, air passes through a layer of materials for pollutants to adhere or "stick." Eventually the adsorbent material becomes saturated or filled with the pollutant and can hold no more. Similar to squeezing a sponge for reuse, the adsorbent material must be refreshed for reuse or disposed.

Activated carbon, silica gel and alumina oxide are common adsorbing agents. The chemical nature of the adsorbent, the total surface area (how porous it is), and pore diameter are given careful consideration before adsorbers are used. If the gas contains particulate matter, the adsorbent bed can become clogged. Some gas is precleaned by a baghouse filter, electrostatic precipitator or cyclone to remove particulate matter before entering the adsorber.

How are Adsorbers Cleaned?

Like squeezing a sponge for reuse, adsorbers can be refreshed too. There are several chemical principles that apply to how adsorbers work. For example, as gas temperature increases, adsorption decreases. As gas pressure increases, so does adsorption. The slower the gas moves through adsorption materials, the more gas removed. These same principles are used to regenerate or refresh saturated adsorbers.

Temperature can be increased to release pollutants from the activated charcoal or the pressure can be decreased. "Steam stripping" uses injected steam to remove pollutants from activated charcoal. Many organic compounds can be condensed, distilled or decanted from captured steam and reused instead of emitted into the air. Charcoal canisters are used to capture emissions from some dry-cleaning machines to remove perchloroethylene, a toxic chemical.

Degreasing, rubber processing, and printing operations sometimes use adsorbers. Toxic or odorous vapors from food processing, rendering plants, sewage treatment plants and many chemical manufacturing process use adsorbers too.

Activated carbon can be made from wood, coal, coconut husks or other nutshells and petroleum byproducts. To activate or make it porous, the material is heated in a chamber with little air. This produces a material with a surface area so great that one gram may have two to five football fields of surface! Activated carbon is often used for controlling organic pollutants such as solvents, odors, toxic gases and gasoline vapors.

Animated depiction of an afterburner functioningAfterburners use a flame enclosed within a chamber. Combustion by-products include water vapor and carbon dioxide gas. They are commonly used to destroy volatile organic compounds. Heat tolerant refractory bricks line the chamber. Pollutant-laden gases are passed through the chamber and burned at temperatures between 1300 to 1500 degrees Fahrenheit. Because of rising fuel costs, heat recovery systems can use waste heat for useful purposes.

Animated depiction of a bag house functioning

In Iowa, fabric filters, commonly called baghouses, are widely used industrial strength "vacuum cleaners." They remove particulate matter found in smoke, vapors, dust or mists.

The filters remove particles from exhaust gases, leaving the particles on the filter while the cleaner air passes through. Collected particulates form a "dust cake" on the filter that is routinely cleaned off by a blast of air in the opposite direction or by mechanical shaking. The dust cake falls into a hopper for disposal or reuse in the industrial process.

Filter bags hang in a sturdy house. Sometimes the house is insulated when cleaning hot gases to prevent corrosive moisture or acid mists from condensing and harming the equipment. Sometimes dozens, even hundreds of cylindrical filters, each eight to 40 feet long may hang in a series of houses at one location.

Filters are made of woven cotton, wool or synthetic materials. Some synthetic materials can withstand high temperatures or are resistant to chemical reaction. Each baghouse must meet the needs of the particular industry process. Gas temperature, moisture content and chemical reactivity decide what filter material is used.

When an industry applies for an air quality permit to install a baghouse, DNR air quality engineers must determine if the baghouse has enough filters and surface area to remove an adequate amount of pollutants before issuing the permit. The DNR also reviews the design to ensure the proposed fan size is adequate to pull or push air through the series of filters. This is determined by the "pressure drop" or the measurement of air resistance across the filters. This can be learned from similar baghouses or calculated using mathematical equations and laws of physics.

A baghouse with a high-pressure drop needs a larger fan or more energy to move dirty air through the filters. Proper design, reflected in a good permit, helps ensure the right equipment is installed to remove enough pollutants. This can reduce the number of operating problems when using the baghouse and prevent possible air pollution violations. DNR staff also reviews the bag cleaning method, hopper design, and other factors.

Baghouse benefits:

Effectively removes large percentages of particulate matter.

Potential concerns:

Bag wear or failure, holes in the house can reduce the 'vacuum' and overall efficiency. Collected dry materials must be carefully handled to prevent release into the air.

A catalyst is a chemical that causes or speeds up chemical reactions without the catalyst itself changing. A catalyst can speed up the burning of organic gases or require lower temperatures to save fuel usage and reduce costs. Platinum or palladium are two elements often used as catalysts. Automobile catalytic converters operate on the same principles to reduce tailpipe emissions.

One concern with catalytic oxidizers is contamination or deactivation of the catalyst material. Particulate matter like soot and dust can coat the catalyst surface. Certain chemicals can combine or react with the catalyst to deactivate it. Sulfur in gasoline for example, reduces the longevity of auto catalytic converters. The U.S. EPA is pushing for reduced amounts of sulfur in gasoline and diesel fuel. This will reduce the 'poisoning' of catalytic converters for better emission controls and reduced sulfur oxides in the air.

Animated depiction of a cyclone functioning

To most Iowans, the word "cyclone" refers to Iowa State University-the home of the Cyclones. But in air pollution control, a cyclone is a device used to remove larger size particles, about 20 microns in diameter, from the air stream. Often, more than 80 percent of particles are removed. More efficient equipment like baghouses or electrostatic precipitators can then remove the smaller particles.

Cyclones are often found at feed mills, crushers, dryers, grain elevators, and even high school industrial arts classrooms. Photos. In industrial uses, cyclones are often used as precleaners for more expensive and sophisticated control equipment such as electrostatic precipitators or baghouses.

Dirty air is forced into the cyclone where it moves in a circular motion in increasingly tighter circles. Centrifugal force causes the larger particles to move toward the outside wall. Like a large, fast moving car attempting a tight curve, the large particles cannot make the turn. They impact the wall and fall to the bottom for collection. Particles are also knocked out of the air stream when they collide with each other. Groups of cyclones hooked together are called multicyclones. Multicyclones are more efficient at removing fine particulate matter.

Cyclone benefits:

Few moving parts, very low capital and operating cost, materials can withstand acids, high heat and pressure.

Potential concerns:

Sticky materials can clog cyclones, hard or sharp edged particles can wear them out. Fails to control ultra-fine particles.

Animated depiction of an electrostatic precipitator (ESP) functioningElectrostatic Precipitators or ESPs have been used in industry for over 60 years. They can collect particles sized 0.1 to 10 microns very efficiently. They are generally more efficient at collecting fine particles than scrubbers or cyclones.

Electrostatic precipitators take advantage of the electrical principle that opposites attract. A high voltage electrode negatively charges airborne particles in the exhaust stream. As the exhaust gas passes through this electrified field, the particles are charged. Typically 20,000 to 70,000 volts are used. A large, grounded flat metal surface acts as a collection electrode. Microscopic particles are attracted to this surface where they build-up to form a dust cake. Periodically, a rapper strikes the plate to knock the dust cake into a collection hopper.

Because no filters are used, ESP's can handle hot gases from 350 to 1,300 degrees Fahrenheit.

A shell or house contains the electrodes, exhaust gases and rapper. The shell must be well built with a rigid frame to hold the components in their proper place. Hot temperatures inside the shell can vary greatly from subzero Iowa winter temperatures outside the shell. Such tremendous temperature differences can cause expansion and contraction to stress joints and welds. Often shells are insulated to minimize temperature differences and prevent gases from condensing into corrosive liquids.

To ensure ESPs work well, engineers must figure out a number of things. "Resistivity" means the particles have some resistance to electricity. This can reduce the effectiveness of an ESP in cleaning exhaust gas. Sometimes changing the gas temperature or changing the water vapor in the gas can reduce resistivity. To make these changes, the correct amounts of water or steam must be added. Sometimes "conditioning agents" such as sulfuric acid, ammonia, and soda ash can be added. Each option can require professional study and review.

The DNR's air quality engineers review air pollution permit applications to ensure the pollution control equipment is the correct size and many other factors. For example, the collection plates must be large enough to clean the volume of gas placed through the ESP. Many technical items must be calculated and agreed upon by DNR engineers and industry professionals before the DNR issues an air quality permit.

ESPs can handle large volumes of hot exhaust gases - beneficial for high-temperature exhausts found at Portland Cement plants, steel industry furnaces and industrial and utility boilers found across Iowa.

Image of a flare operatingCombustion can also be used to control emissions of hydrocarbons and other organic vapors such as chlorine, fluorine and particulate matter (soot). If a process emits volatile organic compounds such as ink fumes from a large commercial printing operation, the fumes can be destroyed or burned.

Flares dispose of intermittent or emergency releases of combustible gases from industrial sources. They are often found in refineries or chemical plants. In a flare, combustible gases are burned in a flame. They are designed to do so with minimal smoke (smoke is made up of dense amounts of soot particulates.)

Animated depiction of a scrubber functioningScrubbers or wet collectors remove particles or gases from the exhaust stream by using water sprays. Gases can be absorbed if they are water-soluble or by adding various chemicals to the spray. Particles of dust or soot can also be captured in microscopic liquid mists. Before the exhaust leaves the scrubber, the liquid mists must be collected before the exhaust enters the public air.

Generally, high-powered scrubbers remove more particles but are more costly to operate due to added energy costs. Scrubbers that remove gases like sulfur oxides, nitrogen oxides or hydrochloric acid depend more heavily on the mechanical and chemical engineering design, not as much on power. Scrubbers are generally better at removing particles than cyclones, but not as good as electrostatic precipitators or baghouses unless operated at high power.

Scrubbers collect microscopic particles by injecting fine mists of water into the exhaust. The moving particles cannot avoid impacting into the droplets, making them easier to collect. Scrubbers must do two things: bring pollutants into contact with the water mist, then remove the airborne water mist. Removing mist sounds simple, but the water droplets are extremely small and the air flows through the scrubber very quickly.

The DNR air quality engineers use physics, chemistry and engineering skills to determine how the system will operate before an air quality permit is issued. The DNR also works with professionals from the business too. Temperature, flue gas and pollutant composition, air pressure, solubility and the chemical reactivity must be calculated to figure out how much pollution will be removed.

Scrubbers come in all shapes and sizes. Sometimes chambers are filled with water spray nozzles. Others use complex systems of baffles, motors, sprays and nozzles. Others use chambers packed with small, odd shaped fill material to increase surface area for particles and water to hit for collection. Some force pollutants through a small nozzle passage to increase the gas flow as it squeezes through the small opening. Here water spray is injected and sheared into fine droplets for the particles to hit.

Scrubbers offer many design options to meet a variety of air pollution control needs. For example, "flue gas desulfurization" scrubbers inject lime or limestone to react with sulfur dioxide gas to form sulfates, which are then removed.

Scrubber benefits:

Can collect both particles and gases and can handle high temperature gases. Fire and explosion hazards found in some dry-collection systems are eliminated with wet collection. Once the pollutants are collected, they cannot escape easily, unlike dry collection systems where dust can be released from hoppers. The water slurry can sometimes be easier to handle than dry dust.

Scrubbers also have disadvantages:

Water and absorbed gases can become very corrosive, so the scrubber must be properly designed to meet each specific industrial process. Because scrubbers use water, high-humidity air leaving the scrubber can cause large water vapor plumes when emitted into cold Iowa winter air. Fog and precipitation can cause local meteorological problems or driving hazards near the industry. And because water is used to clean the air, the dirty water also needs to be cleaned. Settling ponds and sludge handlers are often needed to clarify the water slurry. High-powered scrubbers are costly to operate when using high fan speeds.

Animated depiction of a vapor condenser functioning

Condensers turn a gaseous vapor to a liquid. Any gas can be liquefied if the temperature is lowered enough or if it is pressurized. Condensers cool vapors enough to turn them into liquid.

Dry cleaning machines may use vapor condensers to cool evaporated cleaning solvents such as perchloroethylene for reuse. Without the condenser, the chemical vapor would be lost into the air and more chemicals purchased. Large storage tanks may use condensers to capture evaporated gases and return them to storage. This prevents Iowans from breathing the vapors and allows businesses to use the liquid for its intended purpose, reducing emissions and sometimes saving cost.

Condensers often act as pre-cleaners to remove gas vapors before the air is sent to more expensive control equipment such as incinerators or adsorbers. This reduces the volume of gas needing treatment to reduce costs.