Heavy metals in fruit and vegetables – Organic vs Conventional

Organic farming practices involve multiple steps in the whole system, from cultivation to harvesting, with the end result being a crop that is cleaner and nutritionally closer to how it may be in a natural environment. We may think of natural environments as those before the industrial revolution, when there was less mass production of crops and virtually no pollution.

Fruit and vegetables are particularly susceptible to trace contaminants that accumulate through the soil, including heavy metals. The most concerning of these are lead, cadmium, mercury, arsenic, because of their toxic effects at low levels. For example, lead and mercury are known to cause problems with learning and cognitive function in children [1].

Where do environmental heavy metals contaminants come from?

As mentioned, heavy metal residues can be found in the air, soil and water. They originate from the byproducts of many industrial processes [2] including mining, manufacturing plants that process products and raw materials (e.g. metals) and the burning of fossil fuels (e.g. coal).

Fruits and vegetables are vulnerable to absorbing heavy metals mainly from soil and water supply, because they obtain nutrients from the roots, although it depends on the type of fruit or vegetable. Root vegetables, like carrots and potatoes, are probably more at risk, because they grow in submerged in the soil.

A Chinese study published in 2017 analyzed the levels of mercury in a variety of fruit and vegetable crops grown nearby to large coal-fired power plants [3]. They showed a direct relationship between the distance the crops were to the stations and their levels of mercury. This shows that plants can absorb heavy metals, depending on how much they are exposed to.

Heavy metals in organic vs conventional crops

Organic farming practices are aimed at reducing the exposure of heavy metals and other contaminants, through the use of low impact growing techniques and supply practices. This also involves controlling the location and geography.

A study[4] by the US Department of Agriculture (USDA) has revealed that of the 5 most popular fruits and vegetables, including tomato, carrots, onions, lettuce and potato, the average amounts of toxic metals were generally lower in organically produced samples than conventionally produced crops. This was especially so for cadmium (40% lower) and lead (32% lower).

Another comprehensive review study [5] from Europe published in the British Journal of Nutrition also demonstrated that cadmium levels in organic fruits, vegetables and cereals combined, were on average 48% lower than in conventionally grown crops. This study was the largest of its kind, examining results from a total of 343 separate research studies.

Is it worth testing for heavy metals in organic foods?

Despite these improvements, it is still important to verify whether there is heavy metal contamination in organic foods, because there is often a lot of variation. This comes from the greater amount of natural differences in how crops grow between farms, districts, countries of origin, seasons, and use of different sources in a batch.

This contrasts with conventional farming, which generally aims to have little to no variability in the produce, so as to maximize its consistency, for better yields and profits returns.

  1. Childhood lead poisoning, World Health Organization, 2010. https://www.who.int/ceh/publications/leadguidance.pdf
  2. https://www.epa.gov/international-cooperation/mercury-emissions-global-context#types
  3. Li et al. Scientific Reports, 2019, 7: 46545
  4. https://www.nature.com/articles/srep46545
  5. Hadayat et al. Environ Pollut. 2018 Dec;243(Pt A):292-300. https://pubmed.ncbi.nlm.nih.gov/30193223/
  6. Baranski et al. Br J Nutr. 2014 Sep 14;112(5):794-811 https://pubmed.ncbi.nlm.nih.gov/24968103/