The element chromium (Cr) has two well know faces... but two "personalities":
"Chromium (III) is an essential nutrient that helps the body use sugar, protein, and fat. Cr (III) is a very stable oxidation state for chromium. In this state, the chrome is labile and kinetically very slow to react or form complexes. It is not a strong oxidiser and the human's natural body acidity is enough for the chrome to keep to this Cr (III) state. (reference)"
"Breathing high levels of chromium (VI) can cause irritation to the nose, such as runny nose, nosebleeds, and ulcers and holes in the nasal septum. Ingesting large amounts of chromium (VI) can cause stomach upsets and ulcers, convulsions, kidney and liver damage, and even death.""The main reason why Cr (VI) is so toxic is that one of the reduction products of Cr (VI) is Cr (V). Chrome (V) is a known carcinogen and will lodge in any tissue to form cancerous growths. There are reports that chromium (V) is also a factor leading to premature senility in parts of Russia."
"In the body, the acidity and action of enzymes on Cr (VI) will promote the formation in small quantities of Cr (V). However, as the size of this is normally too large to be adopted by a tissue, the Cr (V) will pass out. The only place where the Cr (V) is likely to lodge is in some of the fine capillaries in either the kidneys, intestines or lungs.""One of the most well known cases of how Cr (VI) impacts the human body is depicted in the movie blockbuster Erin Brockovich. This true story involved a controversial lawsuit against the world’s largest utility, Pacific Gas and Electric Company (PG&E) as a direct result of a massive communications effort mounted by Ms Erin Brockovich."
How likely is chromium to cause cancer?
"Several studies have shown that chromium (VI) compounds can increase the risk of lung cancer. Animal studies have also shown an increased risk of cancer. The World Health Organization (WHO) has determined that chromium (VI) is a human carcinogen. The Department of Health and Human Services (DHHS) has determined that certain chromium (VI) compounds are known to cause cancer in humans. The EPA has determined that chromium (VI) in air is a human carcinogen."
Source: Corrosion Doctors