One important thing to consider is that just like ‘chemicals’, all ‘nanomaterials’ are not the same and whilst some maybe harmful, there are many that are not. It is also worth considering that nanomaterials do come into contact with the body and as have always come into contact with the body due to their presence in the environment (e.g. as a component of air pollution). When we are exposed to nanomaterials, either naturally occurring or man-made, the main routes of exposure are via the lungs, skin or gut.
When particles are inhaled, the lungs act as a size selector meaning that only very small particles can penetrate deep into the lung where gas exchange occurs and particles are removed by the action of mobile cells (macrophages) that eat the particles. Therefore, due to their small size, nanoparticles can efficiently penetrate this region of the lung although what is often overlooked is that true nanoparticles (< 100 nm) rather than larger agglomerates of nanoparticles deposit with high efficiency in the head airways and with lower efficiency in the deep lung. Once in the deep lung (the alveolar region) the particles may interact with the surface of the lung (alveolar epithelium) where, if toxic, it may cause localised damage causing inflammation and possibly scaring of the lungs. As mentioned, mobile cells called macrophages patrol this region of the lung and clear any particles, bacteria or other unwanted debris from the alveolar surface and similarly clear deposited nanoparticles but with lesser efficiency than with larger particles. Just as with the alveolar epithelium, toxic nanoparticles may cause an adverse response in these cells also. Nanoparticles that escape clearance may find their way into the lung tissue (interstium) where they may move to the lymphatic system or the blood, thereby becoming available to the rest of the body and depositing in other regions such as the liver or spleen where they may cause a response.
In relation to the skin, there has been considerable concern that nanoparticles applied to the skin (such as in sunscreen formulations) could penetrate the skin and cause adverse effects. The skin is a complex organ consisting of many layers with the outmost layers being dead cells, which provide an affective barrier to the outside world including particles. A recent in depth review of published studies relating to absorption of nanoparticles through the skin (Poland et al., 2013) found that: “Whilst there are many conflicting results, on balance the literature seems to suggest that absorption of particles in the nano-range through the skin is possible although occurs to a very low degree and that the level of penetration, depending on chemistry and experimental conditions, may be greater than for larger particles”. Therefore, whilst it has been shown that some nanoparticles can be absorbed through the skin, it is at a very low rate and so localised doses either in the deep layers of the skin or more widely in the body should the nanoparticles enter the blood, are likely to be very low.
Exposure of the gut to nanoparticles may occur due to hand to mouth contamination when working with nanomaterials, by particles being cleared from the lungs then swallowed or as intentional additives to foodstuffs. A comprehensive review of absorption of nanoparticles via the gut commissioned by the Danish Environmental Protection Agency (Binderup et al., 2013) reported that similar to dermal absorption, intestinal absorption of certain nanomaterials (e.g. iron oxide, gold and silver nanoparticles) occurred at low levels and was higher for smaller the for larger particle sizes. As stated above, the localised effect of nanoparticles cannot be defined simply on size as some nanoparticles may show toxicity yet many others show low or no toxicity therefore, testing is used to identify hazardous materials as part of approval processes.
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