How does it work?

What Is the Process for Monitoring?

The technology being used is a series of passive diffusive absorption sampling locations to monitor fenceline concentrations for benzene. Basically, we a have series of small tubes filled with an absorbent which hydrocarbons will be absorbed and be retained on the tube over a two-week period. We collect the tubes every two weeks and send to the laboratory to analyze the concentration on the tubes to determine the average ambient air concentration for the two-week sampling period. Another set of tubes is placed immediately after monitoring the next two-week period. Monitoring samples are in place and collecting data for ambient air concentrations every day.

What does Dow do if values are greater than the action level?

We conduct a Root Cause and Corrective Action Analysis (RCA) if rolling annual average benzene concentration exceeds the action level of 9 ug/m3. The RCA process has a specific timeline for completion and reporting to EPA. These steps include:

  • Initiate root cause analysis within 5 days of determining exceedance
  • Implement internal corrective action within 45 days of determining exceedance
  • First sampling results after completion of identified corrective actions must demonstrate effectiveness by results <= 9 ug/m3.

If demonstration unsuccessful, then site has 60 days to submit new corrective plan to EPA for review.

How the Fenceline Monitoring Data Should Be Used

The fenceline data allows sites to have additional insight into their emission sources and their potential impact such that they can take appropriate actions to address the emissions from these sources in the event the annual average Δc for a nearby monitor exceeds the benzene action level. The publicly available data provides transparency and allows for public review. Since samples are taken every two weeks, sites may also be able to identify sources that might lead to elevated fenceline concentrations and can correct issues early, in efforts to avoid exceeding the benzene action level. The data is being provided to the public so that they can stay informed on the status of the site’s emissions sources and the actions the site is taking to address them, as necessary.

How the Fenceline Monitoring Data Should Not Be Used

The benzene action level is not an ambient air standard. The fenceline monitors are not intended to provide a measure of benzene levels in the community. There is no correlation between the benzene action level and any health-based benzene or other hazardous air pollutant exposure standard. The benzene action level does not correlate to a benzene emissions level that presents a risk to the public. EPA did not establish the fenceline monitoring program as a risk reduction step under Clean Air Act section 112(f)(2). Rather, the fenceline monitoring requirements are a development of practices that will provide additional information on the status of emission sources to the public. It is also important to note, that the fenceline monitoring program is not an appropriate tool for monitoring and assessing emergency releases since the data from the monitors are not available immediately.

The fenceline monitors are not limited to measuring emissions from only the site. The passive diffusive tubes may collect benzene from nearby sources that the site does not manage, such as neighboring facilities, roadways, airports, marine ports and from environmental events (e.g., smoke from forest fires). External emission sources may contribute to elevated background readings that are measured by the site's fenceline monitors. Consequently, while this monitoring program is a reasonable means for sites to oversee their emissions sources, there may be situations where the monitors identify benzene emissions that do not originate from and thus does not perfectly isolate site’s emission sources.