Pollutants in Air


The World Health Organization (WHO) estimates that about 8 million deaths occur each year due to air pollution. 4.2 million deaths are from outdoor air pollution and the other 3.8 million are from indoor air pollution. Understanding the key pollutants and their sources is vital to help decrease pollutant levels and stem this trend.

Know the major air pollutants and the way we depict them. This isn't just a local "I made this up" sort of thing. Feel free to notice the same type of list on Wikipedia. Note that I point out the pollutant and its sources.

Carbon monoxide, CO

Pretty horrible stuff here. It is the only pollutant that has no detectable odor whatsoever. This is really bad since we humans will often sense when something is wrong if we see or smell something suspicious. Carbon monoxide is truly a silent killer. Concentration levels above 70 ppm are toxic, while sustained levels above 150 ppm are often deadly.

Sources: CO is primarily produced when a combustion reaction is starved for oxygen. Anytime we talk about combustion we always assume complete combustion of the fuel which means that all the carbon in the fuel is converted into carbon DIoxide, CO2. In order for that to occur you need plenty of oxygen around and available to your reaction. Limit the source (air) and you can end up short one oxygen and your fuel will make carbon monoxide, CO. As an example, below are two balanced reactions burning (combustion) the same fuel, butane.

excess O2:   C4H10 + 6½O2 → 4CO2 + 5H2O

limited O2:   C4H10 + 4½O2 → 4CO + 5H2O

The fact is that both these reactions are "in play" during combustion. The top reaction for complete combustion is favorable when plenty of oxygen is available and the temperature is good and hot (a warmed up engine). The second reaction that makes CO, is much more likely when oxygen is limited (bad ventilation) and at lower temperatures - like a cold engine start.

Nitrogen oxides, NOx

The NOx designation really means both NO and NO2 gases. Nitrogen oxide, NO, is a colorless gas with a sweet-smelling odor... although, you probably won't smell that for long because it is so reactive that it will generally convert to NO2 gas when in an oxygen rich environment (aka: air). NO2 is a reddish-brown gas with an odor that is described as pungent, sharp, biting, acrid, harsh - you get the idea, it is a nasty smelling gas and if you smelled it, you'd try to get away from the source. It has a exposure limit of about 3 ppm.

Sources: Combustion once again is the winner here. Anytime you burn in air, remember you have about 4× more nitrogen gas than you do oxygen gas. So even though a carbon-based fuel burns better than N2, the nitrogen will still react somewhat and make NO gas. Then, if there is plenty of oxygen around, it will fully go to NO2 gas as well. The hotter the source, the better the conditions for making nitrogen oxides. So internal combustion engines (planes, trains, and automobiles) are a primary source for these gases. However, a fully warmed up engine will generally burn the carbon fuel preferentially over the nitrogen gas. This means you get less NOx's during combustion on a full "at temperature" working engine. Percentage wise, more NOx's are formed during a cold start on an engine than at full working temperature.

Smoking: Hey, I can't ignore the fact that even little fires make these toxic gases. Smoking a cigarette is in fact a little fire right there in your hand. Take a drag and bingo - you just made some NOx's (and the afore mentioned CO) and pulled them directly into your lungs. I'll let you do the math on the rest of the questions on smoking - how many cigarettes a day? How many moles of NOx's are you pushing into your lungs to make nitric acid and very reactive free radicals - none of which are considered good for your health. Maybe I'll write a section on risk assessment.

Sulfur oxides, SOx

Strike a match and blow it out. Have a sniff. Smell that? That is the smell of SO2 gas. We even refer to that type of smell as "sulfur-y". The more oxygenated SO3 gas really doesn't have much of an odor at all but will cause great discomfort and difficulty breathing if you enhale it. SOx's are toxic and like all the others on this page - best to be avoided. OSHA has the TWA (time-weighted average) exposure limit on sulfur dioxide set at 2 ppm now (used to be 5 ppm)

Sources: Here's a surprise... ok, not really - combustion again! Burn a liquid or solid fuel and you'll most likely get some SOx's in the product. Gasoline has a trace amount, diesel a bit more, and coal has the most. When coal is burned to produce energy (heat/electricity) it produces a lot of SOx as a pollutant. Yes, this very reaction (burning coal) has no doubt prematurely killed millions of people. Ask London how all that coal burning worked out the late 19th century. Nasty stuff, coal. But remember, there is a bit of sulfur in other "cleaner" fuels. So SOx's ARE getting into our air.

Volatile organic compounds, VOCs

VOCs are basically hydrocarbons and their derivatives that get into the air. There are too many VOCs to even list and show formulas. Their exposure limits have a huge range from as little as 20 ppm up to 2000 ppm. There are 100's of them - although specific ones do show up more than others in the home and office.

Sources: These are all around us - you can often smell them: gasoline, diesel, paint, fingernail polish and remover, pretty much all solvents. Big cities like Houston, Dallas, and San Antonio require vapor recovery systems when pumping gasoline. Those systems are specifically addressing VOCs. It certainly helps, but VOC fragments from fuels can still make it through a combustion engine and be spewed out into the environment. Cold engine starts are the worst - a lot of VOC fragments and even untouched gasoline molecules go straight to the air. A lot of building materials will de-gas and emit formaldehyde (used in their manufacture) and other VOCs into the indoor air. So these things are all around us, indoors and out.

Particulate matter, PM

Teeny tiny bits of matter that float around in the air. They are characterized by their size. PM10 is any air-borne particle that is 10µm in diameter or smaller. PM10's are unsafe beyond 40 µg/m3 and even 20 µg/m3 by other safety standards. Even smaller and more harmful are the PM2.5's. These 2.5 µm diameter particles and less are unsafe at 10 µg/m3 as the limit for exposure. The mechanism is that we breathe these particles deep into our lungs. Our lungs have no real mechanism for expelling tiny little solids embedded in the tissue. These are a primary source for the beginnings of lung cancer and various other unpleasant pulmonary diseases.

Sources: And we have a winner - it's combustion again! Burn stuff and particulates abound. Soot is a major part of particulate matter - it is pretty much everywhere that combustion of larger molecules is taking place. Why large? Well, burning H2 makes only water as a product - no soot. Burning methane is pretty clean as well - it is most all CO2 and H2O. When the molecules start getting bigger and more aromatic (benzene rings), complete combustion is less likely and much of the products are carbon-based particulates - soot. There are others as well. Much of the processing of ores and production of basic raw materials produces ash of some sort which means more PMs. And a relatively new one - microfiber dust particles. The stuff is everywhere thanks to our ever increasing plastic production. Plastics will degrade but often go to fine particulates in the process and get air borne. Synthetic microfibers from clothing puts out tons of PMs.

Ozone, O3

Here's a shocker... ozone is not produced in any real significant quantity directly from combustion. It's not easy to make, and yet, here it is, one of our major pollutants. And, like the others before, it is very toxic to us humans - not to mention, other mammals and even our crops and the leaves on trees. Ground-level ozone, where we live and breathe, that is above 5 ppm is considered deadly. Ouch. So where is it coming from? read on...

Sources: Ozone is what is known as a secondary pollutant. This means that it is produced by chemical reactions that involve one or more other pollutants - in this case, NO2 and VOCs.One of the primary paths is that NO2 gets hit by a UV photon and splits into NO and a free O atom which is a reactive free radical. It's these free radical oxygen atoms that then directly react with oxygen gas and make ozone.

\[{\rm NO_2} + h\nu \longrightarrow {\rm NO + O}\]

\[{\rm O_2 + O \longrightarrow O_3}\]

VOCs can also produce free radicals that ultimately produce the free radical oxygen and leads to ozone. PMs can also aid by providing catalytic surfaces for ozone production. So one set of air pollutants will lead to another - ozone. Now you know why we have "ozone action days" - mix all those pollutants that are generated in mass quantities in a big city with some good ol' UV rays from the sun and you get ozone... time to stay inside (assuming no formaldehyde or radon is there to get you... cough).


I'm good with those 6 for us...

however, there is an elephant in the room!

Carbon dioxide CO2 well, it is not directly going to kill you in even considerably large amounts. Heck, you are exhaling it right now. The list above is also a list of poisons - at least for 4 of them. Saying "poison" is a lot different than saying "pollutant" in my opinion. More on CO2 and how it fits into our climate in chapter 4.



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