Aluminum is a very abundant metal element in the environment, comprising 8 % of the earth’s crust and standing at third position in richness as elements. It is used extensively in many fields of modern life, such as industry, medicine, food, transportation, and living utensil, and may enter the human body from the environment via air, diet, drinking water, food, cosmetic, or medication. Concerns about aluminum toxicity have persisted since the demonstration as a potential neurotoxicant by Doellken more than 100 years ago [1]. This initial finding led to the extensive studies on increment of aluminum concentrations, senile plaque, and neurofibrillary tangles in the brain tissues of patients with Alzheimer’s disease (AD) [2, 3]. Various mechanisms have been proposed for aluminum-induced neurotoxicity, including free-radical damage via enhanced lipid peroxidation and impaired glucose metabolism, disturbed signal transduction and protein modification, alterations in the axonal transport, and abnormal phosphorylation level of neurofilaments [4–6]. Aluminum is relatively stable in the form of alumina (aluminum oxide).

Aluminum oxide, commonly called alumina, is a chemical compound of aluminum and oxygen with the chemical formula Al₂O₃ and is the most commonly existing type of several aluminum oxides, specifically identified as aluminum(III) oxide. It occurs in its crystalline polymorphic phase α-Al₂O₃, in which it constitutes the mineral corundum, a lot of kinds of which form the precious gemstones ruby and sapphire. Al₂O₃ is basically used to produce aluminum metal generally via electrolysis process, as an abrasive owing to its hardness and as a refractory material owing to its high melting point [7]. The nanosized form of aluminum oxide, Nano-Al, has greatly enlarged its utilization in many more industries and fields, such as transparent ceramics, cosmetics, precision polishing materials, special glass, arms, aircraft, electronics, dispersion strengthening, nanocomposites, constituent of rocket propellant, constituent of explosive for military and for mini-bomb killing cancer tissue, solar cell, and drug delivery [8]. As of October 2013, the number of consumer products of nanosized materials has increased to 1628 since 2005 [9], in which alumina is among the most abundantly produced nanosized particles; the market volume is predicted to 5000 tons only in China in 2016 [10]. With this in mind, there has been a recent emergence of concern dealing with the potential for toxicity and the lack of data to substantiate or dismiss these concerns [11–13].

Aluminum oxide was deleted from the US Environmental Protection Agency’s chemicals lists in 1988, but its fibrous form is still on the EPA’s Toxics Release Inventory list, and toxicity of its nanosized particles is still far from being illuminated. With so wide utilization of Nano-Al, many people, production workers, and consumers are being exposed to alumina nanoparticles occupationally and environmentally, and in living condition, concerns regarding their safety and potential toxic effect have complicated their usage. Possible adverse effect of nanosized alumina on human being’s health has not been deeply investigated and elucidated, and the study on aluminum nanoparticles-induced adverse effect is rarely seen.

Toxicological studies [12, 14, 15] have shown increased toxicity of nanoparticles (<100 nm) compared to micrometer-sized particles of the same composition, which has raised concern about the impact on human health from nanoparticles.

In vitro and in vivo studies have shown that ANPs have multisystem and multi-organ toxicity on experimental animals, especially on the immune and nervous system. Thereby, we summarize the immune and neurological adverse effect of Nano-Al in this chapter based on the published data and our studies, even if the investigations were sparse.

Based on breathing air dynamics, nanosized particles can be inhaled more deeply into the respiratory system than large particles and deposited on the surface of the system. The lung tissue is considered the primary target organ for inhaled nanoparticles. Xiaobo Li et al. [16] performed H&E and TUNEL staining to detect pathology and programmed cell death in alumina nanoparticle (Al₂O₃ NPs)-exposed mice lung tissue. They found an inflammation and red blood cells located in the pulmonary mesenchyme; pneumorrhagia characterized by interstitial red blood cell distribution; massive lymphocyte infiltration, especially the subpleural area; lung cell degeneration; and massive bronchial epithelial cell apoptosis. By in vitro study using human bronchial epithelial cell (HBE cell), they also found significant increment of apoptosis (2.24 ± 0.17 %), increased activities of caspase-3 and caspase-9 in Al₂O₃ NP-treated cells, indicating HBE cell apoptosis is initiated by the intrinsic apoptotic pathway, marked damage to the mitochondrial membrane potential, significantly increased cytoplasmic cytochrome c, increased reactive oxygen species (ROS) level, and increased malondialdehyde (MDA). Moreover, they also found that Al₂O₃ NPs significantly triggered downregulation of mitochondria-related genes located in complex I, IV, and V. After having damaged epithelial cells of the lung tissue, alumina NPs may be translocated from the respiratory system to other organs and systems.

Direct input into the blood compartment from the lung tissue is certainly an important translocation pathway of NPs in mammals. However, since predictive particle deposition models indicate that respiratory tract deposits alone may be far from fully accounting for the NP burden in the body, especially in the central nervous system [17], we should consider as well input from other pathways.

Alveolar macrophages are very important first frontier immune cells against foreign materials which are inhaled into the respiratory system. In an in vitro study using human alveolar macrophages (U937) and human type II pneumocytes (A549) coculture treated with aluminum nanoparticle and Al₂O₃ NPs, Laura et al. [18] found that the macrophages as frontier immune cells were more susceptible to the NPs than the epithelial cells, but if the macrophages were not present, the pneumocytes showed significant cell death, indicating that the macrophages actively engulf exotic nanoparticles and interacted with toxicity of the NPs and protected the pneumocytes. The main function of macrophages is to destroy foreign material via phagocytosis. The authors assessed if the macrophages could still phagocytose bacteria named community-acquired methicillin-resistant Staphylococcus aureus (ca-MRSA) after treatment with the NPs and found that the Al₂O₃ NPs did not impair phagocytosis of macrophages to the bacteria, but the Al-NPs did, meaning that the Al-NPs alter the cell function and their higher immunotoxicity than the Al₂O₃ NPs. While ca-MRSA was exposed to Al-NPs, no decrease in bacterial numbers was observed following overnight incubation, indicating that the Al-NPs do not kill this specific strain of bacteria, and the reason of Al-NP-reduced phagocytosis may be due to the Al ions released by aluminum nanoparticles, which chemically alter the cellular environment and finally disrupt the phagocytic process. In a PCR assay, Al₂O₃ NPs alone induced slightly the NF-kB pathway, but the Al-NPs did not show this induction. ca-MRSA alone generated NF-kB pathway activation in cocultured cells, but when the Al-NPs and Al₂O₃ NPs were present with ca-MRSA and together treated the coculture, the cocultured cells did not generate activation of the NF-kB pathway, indicating that the NPs are capable of altering or abolishing the cells’ response to a pathogen via the NF-kB pathway. ca-MRSA infection alone induced inflammatory markers interleukin (IL)-6 and tumor necrosis factor alpha (TNF-α) response in coculture; the NPs alone did not show this effect, but while the cocultured cells were infected by ca-MRSA and the NPs were present, the expression of IL-6 and TNF-α induced by ca-MRSA was abolished, showing that the NPs inhibited ca-MRSA-induced IL-6 and TNF-α expression. In the ELISA, Al-NPs inhibited the secretion of IL-6, IL-8, IL-10, IL-1, and TNF-α too, evidencing the results of the PCR assay.

In a repeated dose exposure experiment reported by Eun Jung Park et al. [19], 6-week-old male ICR mice were acclimatized for 1 week, and then Al₂O₃-NPs were administered orally at a dose of 1.5, 3, and 6 mg/kg for 13 weeks, and the control group was treated with autoclaved water. Blood (approximately 1.2 mL/mouse) was collected from the saphenous vein for biochemical and hemogram analysis, and then the mice were sacrificed, and the brain, thymus, lung, heart, liver, kidneys, spleen, and testis were collected for histological examinations. The levels of aspartate aminotransaminase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH) in blood were significantly different between the Al₂O₃-NP-treated group and the controls. Compared with the controls, the levels of AST, ALT, and LDH decreased in the mice treated with 1.5 and 3 mg/kg Al₂O₃-NPs, but interestingly, these levels were markedly elevated in the 6 mg/kg Al₂O₃-NP-treated mice. In addition, with the same tendency, the accounted number of white blood cells (WBCs) and the proportion of lymphocytes in the WBCs in the mice treated with 1.5 and 3 mg/kg Al₂O₃-NPs were decreased compared with the control group, while those in the 6 mg/kg Al₂O₃-NP-treated group were significantly increased; the proportion of eosinophils in the WBCs in the mice treated with 1.5 and 3 mg/kg Al₂O₃-NPs was increased than in the control group, whereas that markedly decreased in the 6 mg/kg Al₂O₃-NP-treated group. The levels of IL-1β and TNF-α in the Al₂O₃-NP-treated groups did not show significant change compared with the control group, and granulocyte-macrophage colony-stimulating factor and transforming growth factor β were not detected at a significant level in all samples tested. However, the levels of IL-6 and monocyte chemotactic protein-1 increased in a dose-dependent manner. The results of Eun Jung Park et al. indicated that the immunotoxic effect of Nano-Al is complicated.

In a 30-day Al₂O₃-NPs exposure study performed by Li Huan and colleagues [20], 70 healthy SPF ICR mice (3 months old) were treated with Al₂O₃ nanoparticle (13 nm diameter) at 25 mg/kg bw, 50 mg/kg bw, and 75 mg/kg bw as an exposure dose grade and Al₂O₃ nanoparticle (50 nm diameter) at 50 mg/kg bw and bulk Al₂O₃ at 50 mg/kg bw as comparison between nanosized alumina particle and micro-sized alumina particle, by nasal instillation, three times daily, continuously for 30 days. Superoxide dismutase (SOD) activity and glutathione (GSH) content in the spleen tissue and thymus tissue of Al₂O₃ particle-treated mice decreased significantly compared with blank and solvent controls, and in a dose-dependent and particle size-dependent manner, i.e., the higher the dose was, and the smaller the particle size was, the higher the SOD activity and GSH contents were. While MDA content in the spleen tissue and thymus tissue of Al₂O₃ particle-treated mice increased significantly compared with blank and solvent controls, and in a dose-dependent and particle size-dependent manner, the oxidative stress level was increased with increment of doses administered with alumina nanoparticles and with decrement of particle sizes. Inflammatory cytokines IL-1α, IL-1β, interferon-γ (IFN-γ), TNF-α, IL-2, and IL-10 contents in the spleen tissue and thymus tissue increased significantly, indicating immune response in Nano-Al-treated mice was upregulated. The results of this study showed that respiratory exposure to Al₂O₃ nanoparticle could initiate oxidative stress and immune response more strongly than the controls and bigger Al₂O₃ particles, implying Al₂O₃ nanoparticle has higher immunotoxicity than micro-sized alumina.

In an in vivo study with male ICR mice [21], Zhang et al. compared the neurotoxicity of Nano-Al and nano-carbon (Nano-C) as reference of the same particle size and different chemical property and micro-alumina (Micro-Al) as reference of the same chemical property and different particle size. The animals were inoculated intranasally (i.n.) per day with Nano-Al, Nano-C, and Micro-Al at the dose of 100 mg/kg bw as experimental groups, whereas another group of animals that received 0.9 % saline were used as controls. The mice were sacrificed 10 days post-inoculation.

Tested with Morris water maze, treatment with Nano-Al dramatically lengthened the escape latency of animals (Nano-Al vs. control and Nano-C p < 0.01, respectively; Nano-Al vs. Micro-Al, p < 0.05). During the probe trial when the platform was removed, the Nano-Al-treated mice spent significantly less time in the target quadrant (Nano-Al vs. control, p < 0.01; Micro-Al vs. Control, p < 0.05) and exhibited fewer platform crossings (Nano-Al vs. control, p < 0.01; Micro-Al vs. Control, p < 0.05). In Nano-C-treated groups, both measurements were decreased but showed no significant difference from those of the control group (Nano-C vs. control, p > 0.05 for both parameters). In contrast, comparisons between Nano-Al- and Micro-Al-treated groups indicated that mice treated with Nano-Al required longer escape latency, spent less time in the target quadrant, and crossed the platform fewer times (Nano-Al vs. Micro-Al, p < 0.05). In an in vivo study with mice exposed to Nano-Al particles by the respiratory tract [22], Xin Zhang and colleagues reported that only in female mice the neurobehavioral changes and especially depression-like behavior appeared.