Toxics samples are collected over a 24-hour period (midnight to midnight) every 12 days at 18 sites (20 sites before July 1995, 21 sites from July 1995 through July 2000) throughout California. There is usually a maximum of 31 values for a given toxics substance at a given site each year.

Data Use

The toxics sampling network was designed to produce a statewide annual average to support the determination of a statewide risk assessment. Where fewer than 12 continuous months of data are present, we believe that it is seldom appropriate to calculate an annual average. Most of the toxics substances show some seasonal variation, and some substances differ by as much as two orders of magnitude between the high and low periods of the year. If a month's data are missing, the calculated average could be radically different from the real average, the average that would have been calculated had the missing month's data been available.

Since the sampling program was designed to produce statewide annual averages, it is important that the data from a single site not be used too broadly. We have observed that data from sites that are as little as 20 miles from each other can differ drastically. In addition, our knowledge of emission sources leads us to believe that in some instances the area that may be characterized by a sampling site could be as little as a mile or less from that site. As a result, we believe that it is not appropriate to use the data from one sampling site to characterize the annual average of a location beyond the general area of the sampling site.

Please direct your questions about the appropriate use of the data to AQDWeb at the California Air Resources Board.