INTRODUCTION
Winds are well known for their ability to disperse pollution emissions, spreading them over extensive "downwind" areas and by doing so, reducing the pollutant concentration in any one area, including that of the source. However, where two air masses move in opposite directions, pollutants can be trapped between and experience increased concentrations due to this "compression" effect. Though this condition may develop anywhere on a large scale, such large scale compressions tend to be random in location and sporadic in time.

One compression phenomena that is more serious in creating pollution concentrations is the "sea-breeze" or lake-breeze (large lakes) effect. It is more serious because the winds that develop locally along the shoreline have a regular 24 hour cycle and always occur in generally the same area. That is, any concentration compressions will tend to occur repeatedly in the same place, putting residents at increased exposure risk.

This phenomena is examined in detail here, with a focus on how it can specifically increase these risks. In particular, Chicago's O'Hare Airport is examined as an example of a major pollution source that is prone to seeing such compression effects in neighborhoods located between it and Lake Michigan to the east.

SUMMARY
The sea/lake-breeze effect creates a recirculation condition between the inland frontal zone and the lakeshore which traps pollutants that would normally be carried away by prevailing winds. This trapping effect can have substantial impact on pollution concentrations and associated resident exposure, with concentrations in the lake-breeze activity zone climbing to 10-25 or more times greater than would exist without this effect. The effect can and does occur almost year-round, with greatest frontal zone penetration potential in the spring to fall period.

Residents living cast and northeast of the airport between the lake front and O'Hare Airport are generally at greatest risk as they will tend to fall within the lake-breeze pollution concentration zones most often (a combination of southwesterly prevailing winds combined with the lake-breeze effect). Somewhat counter-intuitively, residents along the lake front are also at risk due to being generally always in the recirculation zone. Residents living inland beyond the airport experience less risk, though they can still experience pollutant concentration effects on days when the lake-breeze front penetrates past the airport.

Overall risks increase tremendously when light prevailing wind and high solarization conditions continue to exist over several days, as the combination of lake-breeze and associated land-breeze effects can continue to trap pollutants aid increase concentrations over this period.

Pollution monitoring sites located without regard to this effect may not accurately record pollutant concentration levels under lake-breeze conditions which redirect wind directions away from the monitors.