the tables that a business such as a restaurant provides must be disabled-accessible; this applies to picnic tables as well as other types of seating. Nominally accessible picnic tables can still raise significant hurdles for disabled users. A common difficulty is soft or unstable ground around the picnic table that makes wheelchairs difficult to use. To address this problem, some US states mandate that picnic tables be placed on a concrete picnic pad. Integration with other fixtures Picnic tables are often integrated with other park fixtures, such as shelters and barbecue grills, which may all be attached to a single picnic pad. A bottle opener has sometimes been provided on the edge of the picnic table to dissuade picnickers from damaging the table top by opening bottles on it. Early US Forest Service picnic tables often integrated shelving and cupboards for user convenience, but these proved to be impossible to maintain and were not built after 1941. Integrations with more modern technologies have also been developed: Sonoma State University has developed a solar charging station integrated with a picnic table, sun shade, and weather station, known as a "Smart Table". Problems Placing graffiti on picnic tables, either by carving or tagging, is a common form of recreational area vandalism. Studies in both the United States and Taiwan have found that picnic table vandalism is most likely to occur when the picnic table has already been vandalized. This phenomenon has been explained, using the framework of ecological psychology, as the pre-existing vandalism acting as a releasor cue for new vandals. Consequently, the operators of picnic facilities can best prevent vandalism by ensuring that any vandalism that occurs is addressed promptly. While graffiti is the most common type of vandalism, wooden picnic tables are also sometimes broken up by campers to be used as firewood. Because flat wooden surfaces are vulnerable to decomposition in wet environments, picnic tables have historically often used wood treated with chromated copper arsenate (CCA). In one instance, a worker suffered extreme arsenic poisoning from sawing CCA-treated boards to construct a picnic table. In the United States, CCA-treated wood was in widespread use for outdoor applications from the 1940s until 2003, when an agreement between the Environmental Protection Agency and manufacturers of treated wood ended the chemical's use. Arsenic leaches continuously from CCA-treated wood for the entire service life of the picnic table, which may be up to