The association between high frequencies of Moist Polar air masses prior to high mortality events is consistent with the dew point results. The average morning dew point in Moist Polar air in New York City is lower than for any air mass other than Dry Polar (Table 4 ). Although it may seem counter-intuitive that a ''moist'' air mass has a low dew point temperature, dew point must (physically) be less than or equal to air temperature, so cold air masses typically have low dew points, especially in winter. In general, Moist Polar air masses are relatively uncommon in the cold season in New York, occurring only 7.3% of the time (Table 4 ) and are associated with cold, overcast and often stormy conditions with moist air arriving from the Atlantic Ocean [33] . We also identified a significant relationship between low Moist Moderate air mass frequencies 3-4 weeks before mortality episodes. In an effort to understand this association, we calculated the correlation between Moist Moderate frequencies and the other air mass types during that period. Moist Moderate is negatively correlated with the two driest air masses-Dry Polar (r = 20.47) and Moist Polar (r = 20.31) (Table 4 ). Thus, the significant association between low Moist Moderate frequencies prior to P&I mortality events appears to be a proxy for the cool, low dew point conditions that are common when Moist Polar and Dry Polar air masses are present. On average, cold season dew points are 5.5uC higher in Moist Moderate air masses than in Moist Polar ( Table 4) .