Cyclists who ride on roads are faced with the uncomfortable and unhealthy problem that is the exhaust pipe. Cyclists who travel within traffic or in bike lanes beside traffic notice the impacts of car exhaust and the increase in exhaust inhalation when the exhaust pipe of a vehicle is located on the right-hand side. Although cyclists have been vocal about this issue, there is little science behind it to determine whether a right-sided tailpipe would cause more harm to a cyclist or pedestrian than a left-sided pipe.
Health Affects of Air Pollution
Although there is little research in terms of this specific topic, the topic of air pollution as a whole in relation to cyclists has been studied in detail . A study in the Netherlands conducted a literature review to weigh the risks and benefits of cycling (traffic accidents and pollution versus the benefits of exercising) . It was found that the health benefits of cycling outweigh the risks, but that is not to say that there are not negative health affects from cycling in polluted cities. Studies have concluded that cyclists are at a greater risk of illness due to the higher intake of pollutants from a higher breathing rate during exercise compared to a pedestrian walking [3,4]. Particles travel deeper into the lungs and have more severe health affects . Researchers have also linked cancer  and heart risk  to the air pollution levels cyclists encounter and that diesel fumes are more harmful to cyclists than regular gasoline .
A key factor in determining whether a right- or left-sided tailpipe has a greater impact on cyclists is whether or not the distance from a tailpipe to the cyclist is important and if there is a safe distance. In research investigating whether drivers or cyclists are more exposed to air pollutants, the results are conflicted. Some studies show that cyclists are more exposed than drivers , while others state that cyclists are less exposed [7,8]. Studies that suggested that cyclists are less exposed highlighted cyclists' ability to by-pass congestion and the partial separation of cyclists from traffic [3,9]. Since these studies suggest that a decrease in exposure for cyclists is due to physical separation from traffic, what does this mean for all the roads without separated cycling infrastructure and where cyclists may be stuck in grid-lock with vehicles? Studies also found that pedestrians were at a lower risk of exposure to pollutants than drivers and cyclists . This supports the use of separated bike facilities such as bike lanes or multi-use paths in order for cyclists to avoid traffic and pollutants.
Another study was conducted to compare the exposure of pollutants (PM, CO, CO2, and NOX) between adults and children in both standing and sitting situations . The study proved that children inhale higher concentrations of pollutants due to the proximity of their mouth to the exhaust pipe. The second part of this study concluded that children and adults who are walking, as opposed to standing near traffic, inhale twice as many particles due to increased breathing rates. Although this study does not include cyclists, both findings would give reason to believe that cyclists inhale greater amounts of pollutants than pedestrians; (a) cyclists are closer to the exhaust pipe than pedestrians and would therefore be exposed to a higher concentrations of pollutants, and (b) cyclists are breathing more heavily than pedestrians and would inhale a greater amount (in mass) of pollutants.
Avoiding the Pollutants
There are a number of solutions to avoid or reduce the affects of pollution for concerned cyclists. If living in Montreal or Toronto, McGill University has created a cycling map that provides users with alternative routes to their destination that balance the practicality of the commute with reduced exposure to congestion and pollutants . Companies have also developed pollution masks for cyclists to filter out hazardous particulate matter for biking in urban environments . If cyclists have the option, biking at non-peak traffic hours would also reduce the intake of particulate matter.
One study on tailpipe position was conducted that tested different orientations of tailpipes and their respective exhaust flows and dilution patterns . Unfortunately this study did not compare the difference in dilution between left- and right-sided tailpipes, nor does it test vehicles idling when cyclists are stopped nearby. The correlation between tailpipe siding and health for cyclists is not entirely known and therefore further research is needed. Further, there are gaps in the field of research as to whether there is an optimal safe distance away from sources of exhaust for cyclists to ride. Strong evidence identifying the health risks of tailpipe siding could spur the development of more dedicated cycling infrastructure. The implications of any future research could similarly have profound impacts on vehicle manufacturing with regards to left- or right-hand placement of tailpipes.
 De Hartog, Boogaard, Nijland, & Hoek, Do the Health Benefits of Cycling Outweigh the Risks?, 2010
 Buzzard, Clark, & Guffey, Investigation into Pedestrian Exposure to Near-Vehicle Exhaust Emissions, 2009
 Weichenthal et al, Traffic-related Air Pollution and Acute Changes in Heart Rate Variability and Respiratory Function in Urban Cyclists, 2011
 Kaur, Nieuwenhuijsen, & Colvile, Personal exposure of street canyon intersection users to PM2.5, ultrafine particle counts and carbon monoxide in Central London, UK, 2005
 Goran Petersson, Exposure to Traffic Exhaust, 1987
 Gee & Raper, Commuter exposure to respirable particles inside buses and by bicycle, 1999
 Chang, Hildemann, & Chang, Dilution Rates for Tailpipe Emissions: Effects of Vehicle Shape, Tailpipe Position, and Exhaust Velocity, 2009