Why is Hexavalent Chromium so hazardous

The reason for the toxicity of hexavalent chromium is explained great on Wikipidia

Hexavalent chromium is transported into cells via the sulfate transport mechanisms, taking advantage of the similarity of sulfate and chromate with respect to their structure and charge.

Trivalent chromium, which is the more common variety of chromium compounds, is not transported into cells.

Inside the cell, Cr(VI) is reduced first to metastable pentavalent chromium (Cr(V)), then to trivalent chromium (Cr(III)). Trivalent chromium binds to proteins and creates haptens that trigger immune response. Once developed, chrome sensitivity can be persistent. In such cases, contact with chromate-dyed textiles or wearing of chromate-tanned leather shoes can cause or exacerbate contact dermatitis. Vitamin C and other reducing agents combine with chromate to give Cr(III) products inside the cell.

Hexavalent chromium compounds are genotoxic carcinogens. Chronic inhalation of hexavalent chromium compounds increases risk of lung cancer (lungs are especially vulnerable, followed by fine capillaries in kidneys and intestine). It appears that the mechanism of genotoxicity relies on pentavalent or trivalent chromium. According to some researchers, the damage is caused by hydroxyl radicals, produced during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell. Strontium chromate is the strongest carcinogen of the chromates used in industry. Soluble compounds, like chromic acid, are much weaker carcinogens.

Chromic acid is used in various surface treatments on aluminum, such as chrome plating, chromating and Chromic Acid Anodizing.

In the U.S., the OSHA PEL for airborne exposures to hexavalent chromium is 5 µg/m3 (0.005 mg/m3).