It's not the "heavy" that makes a metal toxic. Lithium and Beryllium are considerably lighter but still toxic to humans. If the metals interact negatively within the human body, it is considered toxic. For e.g. Lead disrupts the functions of the digestive system, nervous system, respiratory system, reproductive system. In addition, it prevents enzymes from performing their normal activities. Lead even disrupts the normal DNA transcription process and causes disability in bones. Lead as such has no physiological role in the body and even smaller levels of lead can cause toxicity.
You can refer to below studies for more information:
- Mahdi Balali-Mood,Kobra Naseri, Zoya Tahergorabi, Mohammad Reza Khazdair, Mahmood Sadeghi, Toxic Mechanisms of Five Heavy Metals: Mercury, Lead, Chromium, Cadmium, and Arsenic, . Pharmacol., 2021 DOI: 10.3389/fphar.2021.643972
- Jaishankar M, Tseten T, Anbalagan N, Mathew BB, Beeregowda KN. Toxicity, mechanism and health effects of some heavy metals. Interdiscip Toxicol. 2014; 7(2):60-72. doi: 10.2478/intox-2014-0009
- Tchounwou PB, Yedjou CG, Patlolla AK, Sutton DJ. Heavy metal toxicity and the environment. Exp Suppl. 2012;101:133-64. doi: 10.1007/978-3-7643-8340-4_6