There are five main reasons why copper (Cu) in sediments is focused on in this review. Firstly, based on global resources data in ^[1][2][1,2], Chile remains the dominant country with 658.2 Mt Cu and world copper resources were at least 1860 Mt Cu (including China). This is mainly caused by human mining for Cu due to the high demand for Cu used in manufacturing industries [1]. Currently, Cu and its compounds have been widely applied in various industries, including textiles, antifouling paints, electrical conductors, plumbing fixtures and pipes, cooking utensils, wood preservatives, pesticides fungicides, fertilizers, etc. [3].

Secondly, due to social-economic activities such as smelting, electroplating, leather production, electronics, agriculture (fertilisers and pesticides), and aquaculture sectors, Cu in sediments can be a secondary source of pollution in aquatic ecosystems. All of the aforementioned activities add to the stress on water and the environment by creating enormous amounts of municipal ^[4][5][4,5] and industrial wastewater ^[6][7][6,7] including potentially harmful compounds such as Cu.

Thirdly, Cu enters the marine environment mostly through rivers and estuaries, it is typically linked to particulate debris which settles and is absorbed into the sediment [8]. As a result, surface sediments constitute the major store and sink for metals and pollutants in aquatic ecosystems ^[8][9][8,9].

Fourthly, human expansion has prompted an increasing release to the environment due to rapid urbanization and industrialization which have led to significant increases in the levels, causing substantial pollution to the aquatic ecosystem. Therefore, it is important to address the role of anthropogenic activities on Cu pollution [8] in relation to many environmental media including sediments which end up as human health risks through different exposure pathways ^{[10][11][12][13]}[10,11,12,13].

Fifthly, this topic has generated a lot of articles in the literature. According to the Scopus database, there were 674 publications published in scholarly journals between 1930 and May 2022 that featured the words ‘copper’ and ‘sediments,’ according to the Scopus database. If the names of the papers included words such as ‘metals’ and ‘elements,’ the number would be substantially higher. If non-Scopus indexed journals were taken into consideration, the number would likely be substantially greater. These figures demonstrate the need to keep an eye on Cu levels in aquatic sediments. It is realistic to expect that monitoring studies will become more common in the future.

Sediment refers to a layer of solid particles on the bed of a water body, which consists of any insoluble particulate matter [14]. These particulate matters could be transported from one area to others by various means, for instance, wind, and flowing rivers. Throughout the fate of a sediment particle, there might be a temporary settlement in between its origin and its final resting place. These sediments may also become settled in a delta at the river mouth or become beach deposits by the action of tides, currents, and waves. Coastal sediments are a major sink for metals of both anthropogenic and natural origin [15]. Under certain conditions, these accumulated metals in sediment may be remobilized, changing the surrounding aquatic ecosystem [16].

Both direct and indirect pathways are likely to play a role in the entry of sediment-bound metals into the human body [17]. Therefore, heavy metal pollution in coastal ecosystem is a serious concern. Despite these concerns, there are few studies that have focused on or have investigated the impact of sediment-bound metals on human health directly. The Cu bound to beach sand particles could enter the human body via inhalation of the sand or sedimentary particles, and direct ingestion via hand-to-mouth action especially by children [18]. Therefore, this implies that humans, especially children playing on beaches, may be exposed to these metal contaminants settled in the sand.

The Hazard Index (HI), Hazard Quotient (HQ), or Target Hazard Quotient (THQ) are used to measure the health risk posed by toxic metals. The HI value reported in urban park soils [18] and kindergartens soils [19] was noted to be comparatively higher in children than adults. Therefore, the risks posed by the ingestion route of sedimentary particles by both children and adults were the highest, followed by dermal contact/inhalation. Therefore, the health risk assessments (HRA) for children should be given a higher priority.

2. Comments on the Hazard Quotients of Children

From the present estimation of the children’s HRA based on Cu levels in the sediments, it is rather far from reality. The ingestion pathway was included in this study with two assumptions that (1) children spend more time on the beach (or muddy sediment areas in the coastal areas), and (2) the sediment-bound metal pollutants could be introduced into children’s bodies via the direct ingestion of small particles by hand-to-mouth action. The first assumption is arguable as the definition for children should be well defined. Most of the papers reviewed din this study did not clearly specify the age groups for their children’s HRA. Children aged 1–2 years old are different from those who are 10–12 years old. The resistance and sensitivity of the children’s bodies are very different between these two groups of ages. Therefore, if those aged 2 to 12 years old are all considered as children, erroneous assumptions could be reached, and as a result, the conclusions would be invalid. Perhaps specifying the body weight of children could reduce the error. However, a similar body resistance and maturity of children between 5 and 12 year old, with a similar body weight of 40 kg, can be assumed. Since obesity among children has become an issue nowadays, the estimation of HQ through the ingestion pathway in a child is somewhat questionable. The present ecological risk of Cu indicated the median concentration of Cu is generally low and not considered a polluted or a low ecological risk. The localized Cu contamination of the 11 papers exceeding 500 mg/kg dry weight is focused on in this commentary section, as shown in Table 1. It can be synthesized that, based on Table 1, there is always a need for mitigation, and the management of an exposure risk assessment of Cu that may be utilized effectively for screening purposes in order to identify significant human health exposure routes. Consequently, all Cu-polluted sediments necessitate immediate sediment cleanup measures. Lastly, there is a need for the ongoing monitoring of Cu’s ecological and health risks in the future.