I read with interest the recently published report, from Imperial College, listing the relative densities of pollution in differing modes of transportation. You can see the BBC summary at the following link - http://news.bbc.co.uk/1/hi/health/4598388.stm.
Why do we care about this? Well, as it happens there is a link between traffic pollution and cancer, according to the report here - http://news.bbc.co.uk/1/hi/health/4368093.stm
The first report, which can be accessed from Imperial College itself here is an investigation into the amount of particulates that occur in the environment around people, the team analysed taxi, bus, cycling, walking, and private car use. Whether traffic fumes scale in a linear fashion with the ultrafine particulates sampled in the experiments is unclear. What is interesting is the findings of the report.
Taxi: over 100,000 pt/cm3Bus: just under 100,000 pt/cm3
Cycling: 80,000 pt/cm3
Walking: just under 50,000 pt/cm3
Car: 40,000 pt/cm3
What interested me most about this was how high the amount for cycling was when compared to walking, and how come it was that the car was lowest.
My initial thought was that it must be location specific. Taxi's and buses often spend much time in clumped together in heavy traffic. Being diesel powered, and diesels (apart from the advanced modern kind) produce many more particulates in operation than their petrol powered counterparts. The fieldwork was carried out in London, at the junction of Marylebone Road and Gloucester Place. Somewhere I lived near many years ago, and regularly cycled to get around. This junction happens to be just outside the London congestion charge zone and almost certainly has more congestion as a result.
Usually bus lanes, which now in London I believe taxis may not use, exist on the side of the road closest to where pedestrian and cycle use occurs.
I think one aspect of research that needs to be considered is the time that people spend in these environments, and circumstances that could affect these results, which are averages after all.
Firstly, as for the major difference between the taxi and bus environments, interesting that people aren't making such a big deal about the high count for the bus! In most taxis and buses the users do not have any control over the ventilation of their quarters. In travelling in a car that does not have air filters fitted I will often open the windows a crack if sitting behind a belching vehicle in front. Most modern cars have filters fitted that remove some of the particles.
Obviously the weather probably can have the biggest affect on the distribution of these particles.
To my mind, however the time factor is the most important issue, and one where, in a city at least where the bicycle comes out in the lead. So your exposure is measured in a density dosage per time. If we assume that the traffic moves at the same sort of rate. Or at least it does in a traffic jam when the pollution is probably the worst (that is a guess or assumption as scientists call it). In a jam the traffic speed is comparible to the traffic, but bicycles very rarely get snarled up in a jam. Let us conservatively say they travel about twice as fast. (I personally cycle 4 times as quickly as I walk without much effort)
So the rate suddenly becomes.
Taxi: over 100,000 pt/cm3/minBus: just under 100,000 pt/cm3/min
Walking: just under 50,000 pt/cm3/min
Cycling: 40,000 pt/cm3/min
Car: 40,000 pt/cm3/min
If there were a decent bus lane, then the bus would fare better as well. I would also be interested to know what the rates of pollution are like on the London Underground network...
Of course there are other factors that will affect this. On the downside of cyclists is the fact that if they are exerting themselves then they may be breathing in more pollutants, less so with the walkers. However, I frequently hold my breath, believe it or not, when cycling and a bus or diesel vehicle pulls away in front of me or otherwise pollutes my air space!
Should you be worried about this then merely consider travelling outside the times of worst congestion. If you are walking or cycling take a different route. Or, as you see in some cities, consider buying a filtering facemask (if they even work, I cannot say myself).
One unknown parameter is however how dangerous each level of ultrafine particle is. If 50K is not dangerous but 80K is dangerous, then cycling is more dangerous than walking, despite the fact that you're travelling much quicker...
Hmmm... Don't follow you Mr Chenepan. Length of exposure is a critical factor, and hence speed matters a great deal. As I've discussed a cyclist will spend _less time_ in the pollution.
So if the exposure rate for a cyclist is twice as much as a pedestrian then as long as the cyclist spends less half the time in the smog then their intake will be less than a pedestrian. (If they are breathing more deeply then they'll be sucking more of the stuff in though so there's a parameter there).
Now, if the pedestrian is smoking, then they'll be getting more pollutants anyway.
You are probably correct that there is a level of exposure that is considered safe. But that is a whole different thing. What we're talking about here is "intake"...
ni eli naiz............................eta arro nago.