Magnesium occurs in seawater and in ores such as dolomite (CaCO ₃ MgCO ₃), magnesite (MgCO ₃), and carnallite (MgCl ₂ KCl 6H ₂O).

In a study published in the journal Food and Chemical Toxicology in 2016, researchers investigated whether titanium dioxide exposure led to an increase in colorectal tumor creation in mice by using a colitis associated cancer model. By measuring tumor progression markers, the researchers found that mice given titanium dioxide experienced enhanced tumor formation in the distal colon.  There was also a decrease of cells that act as a protective barrier in the colon. The researchers wrote: “These results suggest that E171 could worsen pre-existent intestinal diseases.”

There is some concern regarding skin and intestinal absorption of titanium dioxide nanoparticles, which are less than 100 nm in diameter.