Electrical conductivity measurements of ocean air have been made since 1907, and since the electrical conductivity depends on the aerosol concentration in the air, these measurements constitute the longest running record of the aerosol concentration of marine air. The conductivity measurements in the northern hemisphere show an apparent decrease with time, while those in the southern hemisphere are reasonably constant. The apparent increase in aerosol concentration with time in the northern hemisphere, implied by the decrease in conductivity, has been taken by earlier authors to be caused by pollution from the increased industrial development in the northern hemisphere. Ellsaesser (1975), however, pointed out that this interpretation is not consistent with other indications of pollution trends. In this paper an examination is made of the possibility that the apparent increase in aerosol concentration in the northern hemisphere may be due to the increase in the number of moderate-sized volcanic eruptions in the northern hemisphere that have been reported during the past half century. In this paper the historical record of volcanic activity and conductivity measurements is examined. It is found that decreases in conductivity and the corresponding increases in aerosol concentration appear to occur within a few months following moderately large volcanic eruptions. The more numerous moderate-sized eruptions appear to have a greater effect than the relatively few very large eruptions. The good correlation between known volcanic eruptions and the decreases in conductivity suggests that the decreases in fair-weather marine conductivity during this century is indicative of an increase in volcanic activity. Thus the suggestion of Newhall and Self (1982) that the increases in volcanic activity may be only apparent because of better reporting is not fully conclusive. While it is true that the reporting has improved over the past 70 years, our results support the assumption that there has also been a de facto increase in such volcanic activity.