This is an extract from an email exchange I’ve had with an assistant Professor of Clinical Preventive Medicine in. the Division of Environmental Health at Keck School of Medicine of USC.
I found it quite interesting, so I’ve pasted it below.
I think it’s always better to get away from the road vertically if that is an option. I would say just being on the bus and commuting in general will result in a very high exposure; however, if by making a simple decision like sitting on the upper deck you can make a small incremental benefit that is always a good thing. I’m just not sure how dramatic of a difference that will be given the short distance (not aware of any studies measuring this), but should go in the right direction.
We tend of think of 3 stories high and upwards as minimum needed vertically to get away from road emissions (but that is a bit dated information, not sure we know that well. There’s some nice and more recent work starting but not conclusive yet).
The reason is that for most of the very small ultrafine particles emitted from tailpipes (or the bus and surrounding vehicles on roads), because of the way they behave that small elevation difference probably won’t matter much. But for the larger particles that are emitted from non-tailpipe and non-combustion processes, such as brake wear, tire wear, catalyst wear and resuspended road dust which could contain metals and also be a health concern, vertical distance will matter more. Because they are larger and heavier, they tend to settle faster than the ultrafine (smaller) particles at this range we’re talking about..
I haven’t been on a double decker bus in a very long time, I don’t remember if the top deck can also be air conditioned of if it’s open air, but I completely agree with your statement. ” I would expect that the answer is that its worst in the lower deck as PMs are particles and also you are closer to the road and opening doors. I would also expect a variation of this between summer and winter and depending if the windows are open on the top deck or if there is air conditioning and what type of filtration system.”
I don’t know of any research specifically looking at double-decker buses, but I know of some great work that came out of out of London showing significant inflammation, respiratory and cardiovascular effects of walking on high-diesel Oxford street compared to ‘cleaner’ Hyde Park. I modeled my airport ultrafine study after this study design because it’s so powerful.
Here are the links:
NEJM paper on original Oxford Street study (Mc Creanor et al 2007): https://www.nejm.org/doi/full/10.1056/NEJMoa071535
Lancet paper that looks like a follow up study (Sinharay et al 2018): https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(17)32643-0/fulltext
There’s been more metabolomics work done on samples from the Oxford Street study but that gets into biological pathways too much..
Also here is a very nice paper showing spatial gradients of air pollutants as you move away from roads (in 2D). We know that for ultrafines (PM0.1) and larger coarse (PM2.5-10) particles, going a few hundred meters away from the road will result in a large decrease of roadway-related concentrations, but for fine (PM2.5) particles the gradient is not as sharp (these have the longest residence time in the air, they float very well!).
Karner et al 2010: https://pubs.acs.org/doi/abs/10.1021/es100008x