I remember seeing something similar.

As for the study cited by the OP:

1. It measures more black carbon than walking, but it doesn't speak to whether either level (of either cyclists or pedestrians) is harmful. It's possible that neither level comes close to the level required to measure adverse health effects.

2. As noted, it's between cyclists and pedestrians. We don't know the level of pollutants that other commuters like drivers or transit users are exposed to.

3. It doesn't attempt to balance the possible negative health consequences with the health advantages of cycling, which are numerous. I'd be shocked if any possible lung damage wasn't far more than offset by the health benefits of regular exercise, but obviously this is something that warrants further study.

4. It was an incredibly small study of only a few handful of individuals that made no apparent effort to distinguish between the different types of routes taken by cyclist/pedestrian commuters, length of commute, time of day, etc. Most walking commuters that I know tend to live VERY close to work (0 - 1 1/2 mile, 10-15 min walk or so), while bike commuters may travel much farther and for much larger time.

The basic conclusion of the study--that when you're huffing and puffing in traffic, you're inhaling more exhaust than someone walking and breathing less deeply--is pretty self-apparent. But until someone can show that there are serious health consequences that outweigh the benefits of cycling, I'm not going to worry myself over it.