All four peloids currently used in Spanish spas and from Lo Pagán lagoon sediment have been characterized mineralogically and chemically. They range in composition from smectites-rich to complex mixtures or even peat–clay mixtures. The peloids were mixed with artificial sweat and stirred for 1 h (EN 1811:1998 + A1: 2008) at 45 °C (± 2 °C), simulating the thermal-therapy action commonly utilized in the spas. Later the leached extract was analyzed chemically after centrifugation and separation. In the leached sweat extract the concentration of 31 elements, including essential and/or potentially toxic elements, was determined by means of ICP-MS, ICP-AES and AAS.

The principal leached elements are Na, Ca, Mg and K (between 29,000 µg/g and 50 µg/g). The quantity of Na, Mg and K after leaching is related to the composition of the mineral–medicinal water and the mineralogical composition (smectite content) of the raw material. However the leaching of Ca is related more to the presence of calcite and aragonite in the peloid used than to the composition of the mineral–medicinal water.

Other elements are also leached in smaller proportions: Si and Sr (< 45 µg/g, except Sr: 197.5 µg/g in Lo Pagán peloid); Ba and B (< 6 µg/g, except B: 42.9 µg/g in Lo Pagán peloid); Al and Li (< 2 µg/g); Fe, Mn, Mo and V (< 0.4 µg/g, except Fe and Mo: 1.5 µg/g in Lo Pagán peloid); and As, Sb and U (< 0.1 µg/g). Heavy metals such as Cu, Ni, Pb and Zn are ab/adsorbed from the sweat to the peloid leading in most cases to their removal from the leached extract. The other trace elements (Ag, Be, Cd, Co, Cr, Hg, Rb, Se, Th and Tl) are not leached by the Spanish spas peloids or leached at a concentration below 0.05 µg/g in the case of the natural lagoon peloid.

Of all the peloids studied, the highest quantities of elements were leached from the Lo Pagán peloid due to the hypermarine, sodium-chloride-type of lagoon water where the peloids originate.

It is noteworthy that the content of potentially toxic elements (Ag, As, Be, Cd, Hg, Pb, Sb, Se, Tl and Zn) in the leached extracts is negligible (less than 0.05 µg/g).