In the PM10 solution, lead, manganese, and barium concentrations were > 200 (\(\times\)103)㎍/m3, whereas those of arsenic, vanadium, and cadmium were < 90 (\(\times\)103)㎍/m3. Mercury and beryllium were not detected. Heavy metal concentrations in the atmosphere are affected by the geographic environment, season, and temperature [19]. Such concentrations may vary depending on the region even in the same country [20]. Heavy metal components that were measured in this study, however, are also present in the PM10 of other urban areas [20, 21]. Therefore, the results of this study can be applied to other areas.
Among the tested soft lenses, lotrafilcon B that were manufactured from low-water-content ionic materials exhibited the largest heavy metal deposition, and nelfilcon A and nelfilcon A° (high-water-content nonionic materials), and senofilcon C, balafilcon A, and comfilcon A (low-water-content nonionic materials). Among the soft lens material characteristics, the water content and ionic property are known to significantly affect the changes in parameters such as the amount of tear protein deposition and the temperature caused by the surrounding environment [15, 22–23]. The amount of heavy metal deposition in this study exhibited similar tendencies to the amounts of tear protein deposition of soft lenses according to the water content and ionic property[24]. It is also expected that the electrostatic bond due to the attraction between cations and anions can affect the amount of heavy metal deposition.
In this study, there was no relationship between the atomic weights of heavy metals and the amount of soft lens deposition. In the case of ions, however, heavy metals with a + 2 oxidation state, as well known [25–27], showed a tendency to bond well with the lenses. The ionic property of the soft lenses occurred because methacrylic acid was included in the monomers to be copolymerized. In this case, all soft lenses had negative charges, which may have increased the deposition of heavy metals with positive charges.
Lead was deposited on almost all soft lenses, while manganese and barium were deposited on some soft lenses. Lead, which was deposited on the largest number of lenses in this study, has been known to cause oxidative stress, which occurs due to an increase in reactive oxygen species and toxicity. Oxidative stress in the eyes may cause various forms of cell damage, such as protein oxidation, DNA destruction, apoptosis, and lipid peroxidation [28]. If eye tissues are continuously exposed to oxidative stress, it may cause cataract [29].
The WHO air quality standard of lead is < 0.5 µg/m3, based on the annual mean. However, the air quality standard cannot be directly applied to this study because heavy metals were exposed to the soft lenses in a solution that reflected their nature of being worn while hydrated in tears. If a simple comparison is performed using only the values, however, the lead concentration in the PM10 solution was 2.38 ㎍/m3, and 5.50% of lead concentration (0.13 ㎍/m3) was deposited on lotrafilcon B, which was the largest amount of lead deposition. The result was lower than the air quality standard. The WHO air quality standard of manganese (which causes neurotoxicity) is 0.34 ㎍/m3 based on the annual mean [30]. Its deposition amount on lotrafilcon B in this study was 0.01 ㎍/m3, which was also lower than the air quality standard. In the case of barium (which causes neurotoxicity and myotoxicity) [31], the drinking water standard of 7.30 ㎍/m3 was used because a WHO air quality standard has not been established. The barium deposition amount on lotrafilcon B, which exhibited the largest barium deposition, was 0.03 ㎍/m3. Heavy metal deposition amounts on soft lenses in an actual environment, however, may differ because the PM solution used in this study contained PM10 collected for approximately 1.15 hours.
The parameter change of the PM10 exposure group was observed in oxygen transmissibility. There was no statistically significant change in the central thickness, which is related to the wearing comfort and strength of a contact lens, and the refractive power, which is a main optical characteristic. Among the soft lenses, the lead deposition was 5.50% for lotrafilcon B and 4.15% for nelfilcon A°. For the other lenses, heavy metal deposition was less than 3.00%. The oxygen transmissibility change was statistically significant only for the PM10 exposure group among the hydrogel lenses (nelfilcon A and nelfilcon A°), but not for the silicone hydrogel lenses (senofilcon C, balafilcon A, lotrafilcon B, comfilcon A). For hydrogel lenses, the oxygen transmissibility generally increased as the water content increased and the thickness decreased, because oxygen is transferred to the cornea through water. In the case of the thickness, which is calculated as a single value in the denominator, there was no difference between the control group and the PM10 exposure group, indicating that the oxygen transmissibility change was not caused by the thickness. As there was no change in water content in addition to the central thickness, it was predicted that there would be no oxygen transmissibility change caused by PM10 deposition. Some lenses, however, exhibited a statistically significant increase in oxygen transmissibility. These lenses were hydrogel lenses, for which the increase or decrease in water content directly and significantly affected oxygen transmissibility usually, but in this study, there were no significant change in water content. This indicates that PM10 components caused changes that cannot be explained only by the quantitative value referred to as the water content. The water present in a soft lens is bound water combined with the lens material and free water that moves freely in the lens. For the water content, both types of water are measured. Therefore, the proportions of free water and bound water can be changed by PM10 components, and this change may have affected oxygen diffusion and dissolution [32]. In other words, HEMA (Hydroxyethyl methacrylate), which is the basic monomer of a hydrogel lens, has hydroxyl groups composed of hydrogen and oxygen at the end of the molecular structure, and it is possible that these hydroxyl groups formed new bonds by adsorbing heavy metal cations and affected the amount of free water.
In addition to the above changes in the contact lens parameters, even a small change on the contact lens surface caused by PM10 components may affect eye-related symptoms that cause changes in wearing comfort. Examples can be found in studies that report on changes in the characteristics of a contact lens affect the interaction among lipids, proteins, mucins, and electrolytes, which are tear film components, and the changed deposition and denaturation of each component may stimulate the immune response of the eyes or cause osmotic changes [33–37].
In this study, all measurements were obtained after exposing contact lenses to the PM10 solution for 8 hours. Thus, further studies on the effects of repeated or long-term exposures are required. In fact, the replacement of lenses at longer intervals than the specified period was reported as the most common risk behavior among the contact lens users [38]. In other words, the results of the eight-hour exposure may not fully reflect the daily lens wearing time in everyday life. Even though contact lenses were exposed to the PM10 solution for 8 hours, lotrafilcon B (monthly-wear lens) had the largest amount of heavy metal deposition. If these monthly-wear lenses are exposed to heavy metal repeatedly or longer, there might be much more heavy metal deposition and the soft lens parameters change due to the deposition of heavy metals from PM10 may also lead to results different from those of this study.