On the horizontal axis of each graph, we have total traffic-related air pollution exposure, increasing in parts per billion (a part here would be a drop in a billion drops) from left to right.
Consider the graph below, which shows that, in spite of the dramatic increase in the number of minutes per day of cell phone use in the U.S. since the late 1990s, the incidence of brain cancer has not increased and may have decreased slightly.
Assuming an average flow rate of about 1.5 million homes per year, you can eyeball the stock deficit by comparing the mass above and below the zero on the following graph.