https://www.telegraph.co.uk/news/uknews/law-and-order/11381789/The-assassination-of-Alexander-Litvinenko-20-things-about-his-death-we-have-learned-this-week.html
https://www.thoughtco.com/polonium-facts-element-84-or-po-606577
https://www.dw.com/en/just-how-dangerous-is-mercury-anyway/a-16522491
https://www.medicalnewstoday.com/articles/241860.php
https://en.wikipedia.org/wiki/Francium
https://en.wikipedia.org/wiki/Tetraethyllead#Controversy_and_phase-out
https://www.thoughtco.com/worst-elements-on-the-periodic-table-3989077
https://simple.wikipedia.org/wiki/Caesium