| Topic: | Vehicle Air Pollution: Health Effects | |
| Posted by: | Richard Hodges | |
| Date/Time: | 07/03/12 15:01:00 |
| Guy, Carbon Monoxide (CO) A simple toxin, haemoglobin takes up CO molecules with 200x more enthusiasm than oxygen and chokes the blood supply. Causes coma, brain damage and death in high quantities, headaches and nausea in moderate quantities (although this takes quite a long time - more than a day, and up to permanent). A sign of poor combustion, generally not an issue with modern cars & vans with catalytic converters (which oxidise it to CO2). More of an issue with unregulated combustion sources - diesel train, planes, badly maintained gas fires & boilers. Generally no longer considered a major issue, except in very enclosed spaces. Gaseous Hydrocarbons (HC) These are formed in a cool flame, and are created as partially broken-down, partially-oxidised fuel elements. Although they also seem to be related to inefficient combustion , there are reasons why they can't be eliminated (every flame has to cool down at some point). Also like CO, they are oxidised by the catalytic converter (if present), exhausted otherwise. They also break down once exhausted (which can be both good and bad) There's a whole family of these, generally the aldehydes and ethers. There are two effects: - Key in smog (which I'll come back to) - Generally irritants:- think watering eyes, but also dry throat and potentially breathing problems and asthma. Generally considered less of an issue now (because cars exhaust so little) but beware enclosed spaces and less-well regulated producers: like a bus garage with older vehicles. Nitrogen Compounds (NOx) The three combinations: Nitrous Oxide, nitric oxide and nitrogen dioxide. The production method is hard to explain, but concentrations increase with increased pressure (like in a combustion chamber as the fuel burns), and the process is accelerated by high temperatures. Immediately begins returning to the normal N2 and O2, but with the lower temperatures (in the exhaust and then fresh air) mean that it can take several hours to return to a totally safe level. Wind also helps. Again, a catalytic converter helps. Effects are: - Key in smog - Some of it can convert to nitric acid. Not in huge quantities, but especially when combined with irritants like HC, not good for soft tissues like eyes, throat and lungs. Ammonia (NH3) Not normally produced by combustion. The most common cause is badly maintained particulate traps on diesel vehicles (TfL got pulled up badly on this a couple of years ago). It also gets produced at stages through the life of smog. Ammonia is corrosive and particularly damaging to sensitive surfaces. Combined with the other effects, a passable concentration can be a major problem for asthmatics & other breathing problems. Smog A marvellously complicated process (I'm aware of at least thirteen different chemical reactions). Basically, combine the hydrocarbons, the NOx compounds and UV radiation (sunlight), and restrict the supply of oxygen (by not having any wind to stir everything up). What's mainly happening is the oxidation of the HC compounds to CO2 and H2O, which takes in a series of intermediate compounds, all affecting the body in some way. Helping things along, you have the same nitrogen-oxygen processes going on and interacting. All in, some really complicated stuff can happen (for example, reducing the concentration of NO can actually increase the smog levels). Ultimately, it's a combination of the combustion pollution, sunlight, no wind, and a valley or bowl can help. You can see why it's mainly a summer problem, but can also be a winter issue if a high pressure zone sits over the country for a week or so. Normally it's barely an issue, but if the weather's right, the effects start to build. The typical brown tinge is a concentration of NO2. Medical effects are many and varied. You have the irritants, the nitric acid, but also ozone and the dreaded free radicals (ionised molecules that will try to react with anything they come into contact with - like cell walls, DNA). You can see how all these effects together would irritate a healthy body and set it off working overtime to repair cell damage. Particulate Matter (PM10, PM2.5) In the old days, we used to call this soot. Tiny, solid lumps of carbon and carbon compounds. So small, they are easily stirred up and can remain airborne. Compared to the gaseous products I have listed so far, they can survive for a very long time. They are not necessarily visible (not like the old smoky exhausts), and not necessarily from the exhaust (e.g brake pads). The diesel process is more likely to produce them than spark-ignition, and yes, the jet engine is quite prone as well. Immediate effects are what you might expect:- similar to dust and, well, smoke. Dirty, smelly, persistent, and can set you off coughing (in both the short and medium term). As such, they can often be blamed for an issue when other pollutants are the culprit. But there's a longer-term issue which people are (rightly) concerned. Some of the compounds are carcinogenic. Sure, you may not be breathing them in as deep as when you smoke a cigarette, but most people don't consider walking down the street to be a comparable life choice to twenty Rothmans. I'm sure you're aware that carcinogens are a lottery. It could be the first fag that gives you cancer, or the millionth. It took a long time to establish the relationship between cigarettes and cancer, and you put cigarettes in your mouth. When you think of all the other things you breathe in, establishing a measurable causal link between vehicle pollutants and cancer is going to be very difficult - comparing your exposure to exhaust fumes versus a barbecue or bonfire night is arresting. But the risk is undoubtedly real, and it needs to be addressed* *(not just addressing all sources, but the current rain highlights an important point - rain and washing down is a very effective way of removing the particles from the air and flushing them away. Unlike the other pollutants, post-production removal may be as important as preventing production). Sorry if that was long-winded. If it helps, that's about four weeks of reading on an Auto Eng degree, condensed. It's worth recognising how complicated it actually is (understanding how the compounds are actually produced in combustion makes this look like Janet & John!) and how the different issues relate to each other. So in short you have: - Coughing and general dirtiness: particulates - Irritation (eyes, nose, throat and lungs): HC & smog - Tissue damage by corrosion: Nitric acid by NOx; smog; ammonia in certain circumstances. - Asthma and breathing difficulties (from 'a bit irritating' to full attacks): the combined effects of irritation and tissue damage, with all the significant pollutants contributing. - Cancer: Carcinogens, mainly from particulates and with particular concern for the lungs. |