Iodine

Iodine is found the Earth's crust, 5 · 10⁻⁸ % (w/w) [6], and occur naturally in:

• Human thyroid gland [5]
• Kelp [5]
• Sea water (7 · 10⁻⁷ M) [5]
• Underground brines [5]

The naturally occurring isotope is ¹²⁷I (100 %, stable) [6].