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Appel, P.W.U., Na-Oy, Hatsukawa, y:, Osawa, T., Kystol, J. & Sørensen, L.L. 2011: Cleaning mercury polluted mine tailings in the Philippines. Danmarks. Grønlands. Geol. Unders. Rap. 2011/127, 39 pp.

Appel, P. W. U. & Na-Oy L: 2012: The Borax method of gold extraction for small-scale miners. Blacksmith Institute Journal of Health Pollution, 2(3): 5-10

Appel, P.W.U. & Na-Oy, L. 2013: How to mitigate mercury pollution in Tanzania. Journ. Environmental Protection, 4, 1-5

Appel, P.W.U. & Na-Oy, L. 2013: How do we abate the massive release of mercury from small-scale gold mining in Africa. Abstract volume. Compiled by Asfawossen Asrat. 24th Colloquium of African Geology (CAG24) Addis Ababa January 2013, p. 297.

Appel, P. W. U. & Na-Oy, D. L, 2014: Mercury-free gold extraction using borax for small-scale gold miners. Journ. Environmental Protection, 2014, 5, 493-499

Appel, P.W.U.,  Andersen, A, Na-Oy, L.D. & Onos, R. 2015: Introduction of Mercury-free Gold Extraction Methods to Medium-Scale Miners and Education of Health Care Providers to Reduce the use of Mercury in Sorata, Bolivia. J. Health and Pollution. December 2015, Vol. 5, No. 9, pp. 12-17.

Perez, E., Appel, P. W. U. & Koester-Rasmussen, R. 2007. Training of small-scale miners and their families in safe handling of mercury during extraction of gold in the Philippines. Improving access to social services: health services and income opportunities for small-scale miners and their families. Danmarks og Grønlands Geol. Undersg. Report 2007/35, 59pp.