Although whole grain brown rice can be a good source of fiber and contains more micronutrients than white or "polished" rice, it can also contain significantly more arsenic.
Arsenic is a toxic heavy metal released into the environment from mining, pesticides, wood preservation, and coal combustion. It gets into drinking water and cooking water, as well as the water used on crops. The amount of arsenic accumulation can vary depending on the geographic location where it is grown and the farming techniques used to grow it.
In the body, arsenic disrupts cardiovascular, gastrointestinal, and nervous system function, and sufficient amounts may contribute to metabolic disorders including diabetes.
Although arsenic is carcinogenic, modest consumption was not significantly associated with cancer incidence in a review of data from three large population studies: the Nurses' Health Study, Nurses' Health Study II, and the Health Professionals Follow-up Study. However, a potential association was observed with cancer of the bladder, the site most susceptible to arsenic exposure.
The bottom line is to minimize arsenic exposure and take steps to reduce arsenic in rice before cooking. Arsenic concentrates in the bran and endosperm but some can be "leached out" by soaking the brown rice or brown rice products and discarding the water. Unfortunately, this will reduce the content of nutrients as well so be sure to consume adequate micronutrients elsewhere. Using a large volume of water for cooking and rotating white rice with brown rice and brown rice products can help reduce exposure as well.
Biswas, Jayanta Kumar, et al. "Is Arsenic in Rice a Major Human Health Concern?." (2020).
Consumer Reports:
Kumarathilaka, Prasanna et al. “Arsenic in cooked rice foods: Assessing health risks and mitigation options.” Environment international vol. 127 (2019): 584-591. doi:10.1016/j.envint.2019.04.004
Mawia, Amos Musyoki et al. “Inorganic arsenic toxicity and alleviation strategies in rice.” Journal of hazardous materials vol. 408 (2021): 124751. doi:10.1016/j.jhazmat.2020.124751
Zhang, Fan, et al. "Effects of soaking process on arsenic and other mineral elements in brown rice." Food Science and Human Wellness 9.2 (2020): 168-175.