Sample collection

Eight hand sanitizers were collected from different local pharmacies and commercial stores in Muscat and Salalah in the Sultanate of Oman. These samples were coded (A-1 to A-8) to hide the identity of manufacturers and avoid any conflict of interest. Table 1 presents descriptions of the eight hand sanitizers chosen for the study.

Determination of physical properties

   Hand sanitizers were evaluated for the following physicochemical properties for comparison purpose.

Determination of ethanol content in hand sanitizers

Gas chromatography- Flame ionization detector technique was used to test the ethanol content of the hand sanitizers. Briefly, the PerkinElmer 600 GC system fitted with Rtx-5MS capillary column (50 m x 320μm ID x 0.50 μm) was coupled to a PerkinElmer flame ionization detector. Inert gas helium of 99.9999% high purity was used as a carrier gas. The column flow was adjusted to the constant flow of 1.5 mL/min gas, while the total flow was seen to be 77.4 mL/min. The injection temperature as well as the column oven temperature were set to 200°C at a pressure of 10.82 psi with the FID temperature being set to 260°C. The program's progress was started at 40°C with a 1-minute lag time at this temperature and then a 5°C increase in temperature every minute to reach the set 200°C. Two reference standards were used to quantify the specific peaks at which 99.8% ethanol and the solvent (acetonitrile or deionized distilled water) peak appear in the chromatogram. The sample is analysed in duplicate to calculate the average % of ethanol content in hand sanitizers.

General information on hand sanitizers

The gel form of eight alcohol-based hand sanitizers was collected from Muscat and Salalah, the two major cities of Oman. These samples were made in five countries: two each in Oman (A-1 and A-4), Jordan (A-6 and A-7) and Turkey (A-5 and A-8), while a piece in China (A-2) and UAE (A-3). The ethanol content of seven hand sanitizers range as stated on their label, is from 70-76% v/v, whereas the sample from UAE was found to be without an ethanol percentage label claim. The A-6 (Jordan) and A-2 (China) samples were claimed to contain 76% and 75% ethanol, while the label of the rest of the samples showed a content of 70%. The price of hand sanitizers varied from 1.82-4.08 USD. Surprisingly, both the locally made sanitizers (A-1 and A-4) were the most expensive (4.08 USD each), followed by A-2 (3.9 USD; China). The Jordanian samples A-6 and A-7 were the cheapest, with a price of 1.82 and 2.09 USD, almost half the price of the Omani samples. Label of samples revealed that almost all samples contain water and glycerine as a humectant. The label details, including price and country of origin of eight hand sanitizers, are presented in Table 1.

Physical properties of hand sanitizers

The physical properties of hand sanitizers, such as pH, density, and RI, are presented in Table 2. The pH of hand sanitizers was observed in the range of 6.44- 8.06, which is overall consistent with the pH of 100% ethanol, which is very slightly basic at 7.33 (Kosaric et al., 2011). The fluctuation in pH of these hand sanitizers is mainly due to the additives and different percentages of ethanol present. The product with the highest pH was A-2 at 8.06 followed by A-5 (7.42) and A-6 (7.24), which are slightly alkaline but all the other five sanitizers lean towards acidity. This factor can be a purposeful choice made by the manufacturers of these sanitizers, as acidity is shown to denature microbial enzymes. However, the level of acidity isn't high enough to cause severe skin irritation, making these products acceptable to the general public. The near neutrality of these hand sanitizers is imperative to the efficacy and acceptability in the market (Selam et al., 2022).

Density is an important physical parameter to determine the identity of the tested substance. We can assume that the lower the density of hand sanitizer, the lower the ethanol content, since ethanol has a lower density than water (Gumala, Anggraini, & Dewi, 2021). According to the FDA guidelines, hand sanitizers should have a density of approximately 0.85 g/mL (FDA, 2020). In the current study, the density of samples ranged from 0.8664-0.9368 g/mL, where the Omani sample, A-1, had the highest and the Turkey sample, A-7, showed the lowest density. Because the density of all samples is nearly close to 0.85, it could be inferred that all the hand sanitizers comply with the standards.

The refractive index (RI) directly measures the purity or composition of liquids and depends on their density, temperature, and concentration. Ethanol 70% solution (% w/w) has an RI of 1.3652 at 20 ^°C, which is consistent with the obtained results (1.3640-1.3677) given in Table 2 (Wohlfarth, 2017).

According to the WHO guidelines, an optimum hand sanitizer has an ethanol content between 65-70% (WHO, 2009). Each of the products chosen had a specific claim, and the current study aimed to determine if they met this claim and followed the WHO recommendation. Various methods reported in the literature for detecting volatile impurities and assessing the quality and ethanol content of ABHS products include mid-infrared, near-infrared, and Fourier-transform infrared spectroscopy, as well as headspace gas chromatography-mass spectrometry (GC-MS) or GC-FID (Abrigo et al., 2022; Alam et al., 2023; Costa, Haddad, Bigão, Martinis, & S, 2022; Fonseca, Brito, Pimentel, & Leal, 2020). In the current study, ethanol content in the hand sanitizers was determined using GC-FID. The gas chromatograms of analysed samples are illustrated in Figure 1. The results in Table 2, which are computed from the gas chromatograms, show that ethanol content of all the hand sanitizers fell within the range of the WHO recommendation, with a range of 66% to 76%. However, five samples (A-4 to A-8) contained less ethanol content than the label claimed. Only A-1 (Oman) and A-2 (China) hand sanitizers had ethanol contents above the claim at 73% and 76%, respectively. The product A-7 (Jordan) had the least amount of ethanol (65%), lower than the recommended concentration for viral or bacterial inactivation. A study conducted on the ethanol content of seven off-the-shelf hand sanitizer gels, including two biocides and five cosmetics from the Italian market, using gas chromatography, concluded that products with the highest ethanol content have greater antibacterial activity (Berardi, Cenci-Goga, Grispoldi, Cossignani, & Perinelli, 2020). Nisbar et al. employed the GC-MS technique to examine 69 ABHS samples for ethanol or isopropyl alcohol (IPA) content. Out of these samples, 14 samples contained lower alcohol content than indicated on the label, while 4 samples showed the presence of a high amount (5.3 to 19.4%) of methanol impurity (Nisbar, Khair, Bujang, & B, 2023).

Matatiele and colleagues, similar to our study, investigated 94 randomly selected ABHS in the Johannesburg region, employing GC-FID to assess alcohol. Among these, three formulations did not contain alcohol, while the remaining products contained ethanol, 2-propanol, or 1-propanol, either individually or in combination. Notably, 37 (41%) of the hand sanitizers contained alcohol concentrations below 60%. Additionally, ethyl acetate, isobutanol, and other non-recommended alcohols, such as methanol and 3-methyl-butanol, were also identified (Matatiele et al., 2022). In a separate study also conducted in South Africa, de Bruin et al. detected estrogenic activity in 29 out of 60 hand sanitizers, while none of the products met the labelling requirements. Interestingly, while 50 alcohol-based hand sanitizers (ABHS) contained alcohol concentrations of ≥60% v/v, some were found to contain skin irritants and substances potentially hazardous to both human health and the environment (Bruin, Zijl, Aneck-Hahn, & Korsten, 2024).

However, according to Abuga and Nyamweya, beyond alcohol content, numerous other factors play an important role in determining the effectiveness of hand sanitizer products. They proposed a three-dimensional tetrahedron model to manufacture an ABHS. The type and concentration of alcohol, alongside the excipients, formulation, and manufacturing practices, are crucial cornerstones of their model (Abuga & Nyamweya, 2021). Many studies advocated for the utilization of ABHS in the fight against COVID-19. It was pointed out that the efficacy of ABHS depends upon various factors, including but not limited to their proper application, applied volume, contact time, viral load, manufacturing techniques, selection of active ingredients, handling procedures, and ensuring the safe deployment of hand sanitizers against the targeted pathogen (Saha, Khadka, & Das, 2021; Singh, Joshi, Samuel, Patra, & Mahindroo, 2020).

In 2022, Manuel et al. conducted a comprehensive study to evaluate the efficacy and safety of ethanol-based sanitization products, particularly those of uncertain quality, and the utilization of bulk refillable dispensers amidst the COVID-19 pandemic. Their findings unveiled concerning revelations: a prevalent occurrence of substandard alcohol levels, noticeable visual impurities, and alarming concentrations of potentially hazardous chemical impurities in these products. This research unequivocally underscores the inherent risks linked with the use of subpar alcohol-based sanitizers and the common practice of bulk refilling (Manuel et al., 2022). Another interesting study evaluated the quality of 310 marketed ABHS products registered with the FDA through GC-MS technique. Surprisingly, findings revealed that a significant portion, 104 products (33.5%), failed to meet the declared alcohol content as per their labels. Ethanol-based sanitizers fared worse in both overall assay and impurities, with a failure rate of 84.3%, compared to isopropanol-based counterparts, which showed substantially lower rates (11.2% and 6.2%, respectively). These results underscore the critical importance of stringent testing protocols in ensuring the production of high-quality sanitizer products (Stafford et al., 2023).

While the COVID-19 pandemic may have ended, it has left a lasting lesson on the critical importance of hand sanitization in mitigating the spread of infectious diseases. This global crisis has significantly altered public perception and transformed hygiene practices, with alcohol-based sanitizers emerging as indispensable tools in the fight against the coronavirus. The heightened awareness of the public persists even in the aftermath of the pandemic, as many continue to prioritize hand hygiene through the use of alcohol-based sanitizers. However, it is imperative to assess the continued relevance and safety of alcohol-based sanitizers in the post-pandemic era. According to Phillips et al., there was a notable 72.5% rise in exposures to alcohol-based sanitizers between 2019 and 2020, as per data compiled by Texas Poison Control Centers. Fortunately, the majority of these exposures did not necessitate medical attention, and there were no reported deaths among the cases documented in Texas for the specified period (Phillips, Schulte, Smith, Roth, & Kleinschmidt, 2020). In another study carried out in the United States, researchers examined ethanol and organic impurities in 31 children's ABHS products. The findings revealed that seven of these products surpassed the impurity thresholds established by the FDA's recommended interim limits. Notably, benzene levels reached up to 9.14 ppm, while acetaldehyde and acetal concentrations reached as high as 134.12 ppm and 75.60 ppm, respectively. Furthermore, the total measured alcohol content varied significantly, ranging from 52% to 98% across all tested hand sanitizers. This range extended from 39% below to 31% above the labeled concentration (Gloekler et al., 2022). Corcoran et al., recently published findings from a single-center, retrospective review examining pediatric (<19 years) hand sanitizer ingestions at their Wisconsin poison center in the US spanning from May 1, 2020, to January 28, 2022. During this period, they received 801 calls related to hand sanitizer exposure, leading to 140 children being referred to a healthcare facility due to hand sanitizer ingestion. Among these cases, 88 (63%) underwent measurement of methanol and/or ethanol concentrations. Interestingly, no child exhibited detectable methanol concentration, while 78 underwent ethanol testing, with 12 showing detectable ethanol concentration (Corcoran, Feldman, & J, 2023). 

Therefore, there is a pressing need to design and develop quality formulations that prioritize safety by minimizing flammability risks, ensuring efficacy in antimicrobial action, and mitigating potential skin irritations and other adverse effects associated with the frequent use of alcohol-based sanitizers (Abuga & Nyamweya, 2021; Jing et al., 2020). Furthermore, user awareness on safe handling of ABHS could help in preventing their potential deleterious effects such as allergies, skin irritation, lung injury, fire hazards, and toxicities (Saha et al., 2021).

Figure 1

GC chromatogram of hand sanitizers showing adistinct peak for ethanol