OSCaR report on cancer rates near Bullseye Glass

The OR state cancer registry (OSCaR) released a report today examining the rates of lung and bladder cancer in census tracts near the Bullseye Glass factory in southeast Portland (OR). This district of the city is the location of elevated exposures to cadmium and arsenic measured by the Department of Environmental Quality (DEQ), and understanding whether those exposures pose a threat to human health has been a pressing issue.

The OSCaR report is based on comparing observed rates of cancer in census tracts near the site of the metal emissions with observed rates of cancer in the county overall. Specifically, the study calculates Standardized Incidence Ratios (SIRs) for both lung and bladder cancer. Those ratios divide the observed number of cancer cases in sites of exposure by the number of cases expected based on the number of people living in those sites (again using the county as a whole to best estimate that baseline rate). SIRs greater than one indicate an increased rate of cancer in the study area, while an SIR less than one suggests cancer rates in a study area are actually below baseline rates.

Here are the SIRs for the census tracts near the Bullseye plant:

As you can see, the ratios are close to one for both cancers, whether including just the census tract that includes the plant, or adding the adjacent census tracts to the east. That is good news!

You may notice that the confidence intervals for the study are large. These are 95% confidence intervals; they essentially tell you that, for example, the actual SIR for bladder cancer in tract 1000 has a 95% chance of falling between 0.3 and 3.1 given the number of cases measured. That number of cases is what makes the confidence intervals large; in this example it is based on four actual cases reported among residents over the five-year study. You can see why the CIs would be wide; having just one more (or one fewer) case of bladder cancer in that tract would change the rate by 25%. Since the number of cancers reported is small, the confidence intervals are broad. I should point out that this is a good problem to have - the cancer rate is hard to estimate because there are not many cases! From a statistical point of view, if the confidence interval on an SIR includes the value 1.0, there is no statistical evidence that the rate of cancer is different in the study area. That is clearly the case in these data. Even better, the actual estimates of the SIR are all close to one; this is very reassuring.

There are limitations to this analysis, of course. First off, given the low number of cases, it is just not sensitive enough to pick up very small changes in the cancer rate. The differences between the estimates above and exactly one are small compared to the confidence intervals, yes, but realistically the study does not have the statistical power to pick up a change in rate less than about two-fold (for bladder cancer) or 1.5-fold (for lung cancer) in the study area. There just aren't enough people living there (and not enough reported cases) to be more precise. In addition, the conclusions are limited to the two cancers studied, though those are the cancers known to have the strongest links to arsenic and cadmium exposure.

I'm personally very heartened by these results, for a few reasons. First off, because the data were collected for cases that pre-date the release of the DEQ emissions testing data, we don't have to try to adjust for reporting biases. Second, the exposure thresholds for cancer tend to be among the lowest exposure thresholds in the health-effects literature; you would expect to see cancer effects at lower exposures than for acute diseases caused by heavy metal exposures. I don't want to give you the impression that we're measuring 1 in 10,000 risks well in this study; the total population in the study area was 13,725 - so the result of an increase of 1 in 10,000 would be just over one additional case for a given cancer. But I'm encouraged that the predictions that come from examining the DEQ emissions levels hang together nicely with this result from cancer rates; any additional risk for human health effects looks to be small (or less) for cancer; hopefully that conclusion holds up for other types of disease.

Finally, I want to congratulate OSCaR for the study; I imagine that running a cancer registry is rather low-profile and out of the public eye until you really need that registry to answer a question like "Should we be concerned about cancer from these arsenic and cadmium emissions?" Then it's really nice to know that the data have already been collected, and within a couple of weeks of the first media reports we have a direct, evidence-based answer to that question. Thanks to everyone at OSCaR!