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Release of Micronized Copper Particles from Pressure Treated Wood Products
Citation:
Platten, W., T. Luxton, T. Gerke, S. Harmon, N. Sylvest, K. Bradham, AND K. Rogers. Release of Micronized Copper Particles from Pressure Treated Wood Products. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-14/365, 2014.
Impact/Purpose:
The current report provides the first published data on the potential for the release of copper nanoparticles from pressure treated lumber. The report examines release under a variety of conditions and provides for the first time a detailed analysis of the speciation of copper released from the lumber during intended use. The report was a deliverable product for CSS Project 1.2.2.
Description:
Micronized copper pressure treated lumber (PTL) has recently been introduced to the consumer market as a replacement for ionized copper PTL. The presence of particulate rather than aqueous copper raises concerns about the exposure of humans as well as the environment to the particles. Two common pathways of exposure, leaching during contact with water and transfer during physical contact, were investigated to gage potential human and environmental risk during intended use of the product. Characterization, leaching tests, and wipe tests were conducted on two representative formulations of micronized copper PTL (micronized copper azole or MCA) to quantify the levels of copper present in the treated material and the amount of copper released during use as well as to determine the form (particle or ion) of the copper after it was released. Additionally, an ionized copper pressure treated wood (alkaline copper azole or ACA) was tested for comparison. The characterization showed that copper carbonate is the primary particle form in the MCA treated wood, but other forms are also present, particularly in the MCA-1 formulation, which contained a large amount of organically complexed copper. Microscopy showed that MCA-1 contained particles roughly half the size of MCA-2. The leaching results indicate that mostly (> ~95%) ionic copper is released from the MCA wood and that the particulate copper that was released is attached to cellulose and not free in solution. A small number of particles were captured separate from the cellulose on a 10 kDa filter, but the quantity amounts to less than 1% of the total copper leached from the wood during the test. Comparing the MCA wood to the ACA wood, MCA released significantly less copper than the ACA, leading to a lower potential impact on the environment, though ACA has already been shown to have a negligible impact. The wipe tests were a surrogate for hand contact with the treated wood, developed by the Consumer Product Safety Commission for gauging exposure of children using playground equipment constructed with PTL. The results show that the MCA and ACA wood release approximately the same amount of copper with each contact and that the amount of copper released is high initially, but decreases to a constant level after being wiped 2-3 times. The boards which were left outdoors, exposed to the elements, had a higher plateau level than those tested indoors, leading to the conclusion that the exposure, likely to precipitation, causes migration of the copper to the surface. During the initial period of high release, contact with the wood causes far more depletion of the copper than any type of environmental exposure. During testing, MCA-1 released slightly more copper than MCA-2, possibly due to the smaller size of the copper particles in the MCA-1 formulation or the increased concentration of organically complexed copper. Based on the transfer levels observed in this study, children under the age of 8 years could be impacted by the levels of copper, likely experiencing mild symptoms. Severe acute and chronic symptoms are unlikely during normal use. In summary, the particulate copper released from micronized copper PTL constituted a small fraction (< ~5%) of the total released. The total copper released was less than or comparable to the current aqueous formulation available to consumers.