Inventory of Engineered Nanoparticle-Containing Consumer Products Available in the Singapore Retail Market and Likelihood of Release into the Aquatic Environment

1. Introduction

2. Methods

3. Results and Discussion

3.1. Types of ENP and Their Application in Consumer Products in the Singapore Retail Market

In the first step, reports and literature [3,8,9,20,22,28,29,30,31,32,33,34,35,36,37,38,39,40,41], were reviewed to identify the most frequently used ENP and their applications. Ag, TiO₂, Al/Al₂O₃, ZnO, SiO₂, ceramics, iron/iron oxide, carbon nanomaterials, and nanoclays were identified as the most frequently used ENP in consumer products. The identified ENP along with its function and consumer product application are summarized in Table 1. These identified uses of ENP types guided our nano-inventory activities. In our Singapore market inventory, we evaluated 1432 articles. Of these, 431 were “confirmed” (138 articles) or “likely” (293 articles) to contain ENP. The results are summarized in Table 2. These ENP-containing products occurred in nine different categories: (1) sunscreen, (2) cosmetics, (3) health and fitness, (4) automotive, (5) food, (6) home and garden, (7) clothing and footwear, (8) eyeglass/lens coating, and (9) electronics (see Figure 1). A total of 27 different types of nanomaterials were identified in these articles. In particular, ENP were widely utilized in the category of Food (175 articles), Health and fitness (108 articles), Cosmetics (68 articles), and Sunscreen (23 articles). Of the total of 431 articles, 34 contained more than one type of nanomaterial (e.g., ZnO, TiO₂, and Al₂O₃). In our nano-inventory, it is relevant to note that housekeeping cleaning products supplied in the Singapore market such as laundry detergents or air fresheners have no detailed information regarding the products’ ingredients. Thus, it was not possible to assess if these products contained ENP or not.

Table 1. Applications of ENP in identified consumer articles.

Table 1. Applications of ENP in identified consumer articles.

ENMs	Function	Application Areas	Reference
Ag	Antimicrobial protection Conductivity or electrical properties	Textiles, food packaging, medical devices, water treatment process, surface coating, electronics such as transparent conducting films, transparent electrodes for flexible devices	[3,8,9,20,28,29,30]
TiO2	Absorption of ultraviolet radiation Catalyst	Electronic devices, sunscreen, cleaning, water treatment, solar cell, health and fitness	[8,9,20,28,29]
Al2O3	Extremely fine powder with great capability of polishing Absorption of light Antimicrobial protection Adsorbent	Cleaning, cosmetics, water treatment, coatings, food additives, catalysts, ceramics, electrical insulators	[8,9,20,28]
Al	Dispersion strength catalyst	Cosmetics, catalysts, coating, optical biomaterials, drug delivery	[8,20,28,31]
ZnO	Sun protection Antimicrobial protection	Sunscreen, cleaning, paints, cosmetics, food packaging, personal care products/sprays	[8,9,20,28]
SiO2	Extend life of paints and coatings	Paints, coatings, food packaging	[8,9,20,28]
Zr/ZrO2	Good absorbability and hydrophilicity Bio-corrosion resistant and bio-compatible	Drug delivery, water treatment (membrane, ion exchanger), coatings, fuel, battery, pigment, abrasive material, medical implant	[8,28,32,33]
Hydroxyapatite	Biocompatibility	Therapeutic and/or diagnostic agents coatings, drug delivery, sensor, biomaterials	[8,20,28,34,35,36]
Ceramics	Filtration with great rejection efficiency Hardness and strength	Home and health/filter, paint, personal care products, cosmetics, food and beverage, coatings	[3,8,20,28]
CeO2	Catalyst Sun protection Optical property	Coatings, paints, automotive/fuel catalysts, biomedicine	[8,28,37,38,39]
Fe/FexOy	Hardness and strength magnetic and catalytic properties	Food and beverage, home and health/sporting goods, environmental remediation, water treatment, catalysts	[8,9,28,40]
Carbon nanomaterials (C60, carbon black, carbon nanotubes (CNT)	Hardness and strength Light with high density Conductivity Metallic property Mechanical and Magnetic property	Probing electronic devices, catalysts, adsorbent in water treatment, detection devices, molecular filtration membrane, detection sensor or probe, automotive, sporting goods, clothing, food packaging	[3,8,9,20,22]
Nanoclays	Hardness and strength Improvements in mechanical, thermal, flame resistance, and barrier properties	Food packaging, automotive, cosmetics and toiletries, environment and water treatment, flame retardants, medical devices, packaging, paints, pigments and coatings	[3,8,20,28]
Organic nanomaterials	Organic nanomaterials are widely used in a variety of field, including vitamins, anti-oxidants, color agents, flavors, preservatives, drug delivery, cosmetics, nutrients and supplements.		[8,20,28]
Other nanomaterials	Many other nanomaterials are being increasingly used in the commercial field, including Cu and CuxOy, Ti, metal nitrides, and alkaline earth metals. Quantum dots composed of metal (oxide), or semiconductor materials with novel electronic, optical, magnetic and catalytic properties are used in medical imaging, diagnostics and security printing at increasing rates.		[3,8,20,28,41]

Figure 1. Distribution and category of products with confirmed or likely use of nanomaterials.

In the category of Health and fitness (or personal care products) and Food, TiO₂ and SiO₂ were widely used, namely in toothpastes, beverages, instant noodles, and sweets, either as a color additive or an anti-caking agent, respectively. The ingredients of the products had no indication if they were in the nanoscale or not. However, Weir et al., (2012) [42] reported that approximately 39% of food-grade TiO₂ particles counts were less than 100 nm in at least one dimension and that they readily dispersed into water as fairly stable colloids. Even though this report was limited because the percentage only referred to particles smaller than 220 nm (thus ignoring many of the larger, white TiO₂ particles), it confirmed the presence of nanoparticles in food-grade TiO₂. Peter et al., (2012) [43] observed the presence of nanosized SiO₂ during the digestion of foods containing SiO₂ as a food additive using SEM. Thus, for the purpose of this study, which aims to estimate ENP concentrations in different aquatic environments, toothpaste, beverages, instant noodles, and sweets were categorized as likely to contain ENP. Ag was used in many consumer products as an antibacterial agent, including: cleaning detergent products in the category of Home and garden (4), household electrical appliances (4) in Electronics, toothbrush (1) in Health and fitness (personal care), and socks (1) in Clothing and footwear.

Of the 431 articles with confirmed or likely use of ENP, producers declared the use of nanomaterials or nanotechnology for 69 of them, but no indication on the type of nanomaterials was provided. They were from the sectors of skincare/makeup/cosmetics (18), health and fitness (23), automotive (22), and clothing (6). Since it was not possible to directly determine the type of nanomaterial, they were not included in the modeling of emission quantity, which may lead to a slight underestimation of quantities.

Most of the commonly used nanomaterials listed in Table 1 were also identified in our inventory of the Singapore retail market. Some others, however, were not found, namely: CeO₂, Quantum dots, Zr/ZrO₂, and Hydroxyapatite. This may be attributed to insufficient information about an article’s ingredients, lack of penetration of those types of nanomaterials into the (Singapore) market, or the fact that these may be niche products not found in major retail and cosmetic stores.

3.2. Release of ENP during Use and Disposal of Consumer Products

The product types and the ENP contained therein were assessed with regard to their likely release patterns into the aquatic environment during the use or disposal of the articles. The various product categories were assigned to the following release paths:

Sunscreen, cosmetics: Both the review results and nano-inventory results indicate that the main ENP in this category includes TiO₂ and ZnO, which was expected because sunscreen use was likely to be significant in a tropical area such as Singapore. In this category, both TiO₂ and ZnO, are suspended in liquid. They can be directly released during the use and wash-off process. Most of them will be released into wastewater, while negligible amounts will go into surface water (no swimming in Singapore’s fresh water reservoirs and lakes).

• Health and fitness (personal care): TiO₂, SiO₂, metal oxides, and nano carbon are widely used in this category. Products include: toothpaste, shampoo, shower gel/milk, facial cleaning gel, and body lotion with UV protection. The use of products in this category will lead to direct release of ENP into the wastewater. For example, TiO₂ can be easily released from toothpaste during tooth brushing. In addition, SiO₂-containing facial cleaning gel and shampoo have great potential to release particulate SiO₂ into wastewater.
• Automotive (i.e., tire, polish and wax, coating, fuel additives): This category was found to be important primarily because of consumables such as tires, which are worn down during their lifetime. This can lead to direct release of ENP into the air and onto road surfaces, from where ENP can go directly into the aquatic environment via road runoff into surface water and in small quantities by air transport and dry deposition. ENP from maintenance products and accessories, such as car coatings or wax products, can be released into road runoff but also into wastewater during their application at private homes. The predominant ENP in this category include carbon black, ceramics, and SiO₂.
• Home and garden (e.g., coatings, cleaning products): TiO₂, Ti, and Ag are widely used in this category. Products in this category are relevant in terms of potential for direct release of ENP into surface water by outdoor use, and into wastewater during indoor use and disposal of products.
• Clothing and footwear: Ag is used in nano form in textiles as an antimicrobial agent. Wearing clothing with Ag is expected to lead to only minimal direct release of Ag into the aquatic environment. The main release will occur during washing of clothing containing nanoparticles of Ag into the wastewater systems [44]. The amount of ionic and particulate Ag released from the fabrics depends on the type of Ag incorporation into the textile.
• Eyeglass/lens coating: This is a small-scale use category. ENP may be released into wastewater when washing glasses but only if the ENP were non-permanently coated on the surface of the lenses. The main ENP involved include TiO₂, SiO₂, and polymer thin films, which are all permanent coatings.
• Construction materials, paint, and coating: Most nanomaterials being used in this category are fixed in the products. Some ENP such as Ag will be released in ionic form. Environmental degradation over time and rainfall flushing during heavy storms may cause small quantities of ENP to be released into the aquatic environment. Most of the released nanosized particles are expected to go into surface runoff.

For each of the 431 articles confirmed or likely to contain nanoparticles, the likelihood of releasing ENP into the aquatic environments was assessed based on the above considerations. Of the 431 products assessed, 324 (75%) were identified as likely to release ENP into the aquatic environment and 107 (25%) were unlikely. Table 3 shows the types of matrices that contain ENP in the identified products, the expected release pattern, and the assigned release factor into the aquatic environments. The “release factor” is defined as the fraction of ENP released “down the drain” compared to the total usage quantity. The value of these release factors were identified in the literature and estimated for the purpose of this study. Assumptions behind estimates are listed in the reference column. For example, as women may use facial cottons to remove makeup products such as foundation cream and concealer before face washing, the assumption was made that 70% of the substance was transferred via the facial cottons into solid waste. The remaining 30% will then reach the drain during face washing. In some cases, the value of the release factor was set to 0%, which means these products are not likely to release ENP into the aquatic environment despite the presence of a hypothetical release pathway. For example, nano-capsules and nano-hyaluronic acid in skincare products can be adsorbed when they come into contact with skin. After adsorption, they will lose their nano form and thus the release factor for these skincare products was assumed to be 0%. Also products such as rackets and toothbrushes which have a permanent coating would not release ENP through their lifetime. TiO₂ and SiO₂ contained in beverages, snacks, instant noodles, and sweets were assumed to enter into the aquatic environment after passing through the gastrointestinal tract, whereas food that was thrown away would land in solid waste treatment and thus not enter the aquatic system.

Table 2. Product category vs. ENP within 431 products assessed.

Table 2. Product category vs. ENP within 431 products assessed.

Category	#	TiO2	ZnO	Pt	Nano–hyaluronic acid	Si	Cu	Nano capsule	Nano vitamin E	Nano–lipobelle E Q10	Nano Liposome	Au	Ag	SiO2	MnO2	Ti	Ceramic	Iron oxides	Al2O3	Alumino–silicate oxides	Copper oxides	Nano Collagen	C60	Carbon nanotube	Carbon nanofiber	Nano Fiber	Carbon black	Nano glass	Unclassified
Sunscreen	23	19	12
Cosmetics	68	22	2	5	4	1	1	2	1	2	1	12		1	1			4	4	1	1	2				1			19
Health and fitness	108	69	1									1	1	18		2		1					1	1	1	2			22
Automotive	31	1												1			5										1	1	22
Food	175	11	5											159
Home and garden	10	6											4			1
Clothing and footwear	4												1																3
Eyeglass/lens coatings	1																												1
Electronics	11												4			4	1												2
Total	431	128	20	6	4	1	1	2	1	2	1	13	10	179	1	7	6	5	4	1	1	2	1	1	1	2	1	1	69

Table 3. The type of matrix containing ENP, release pattern and release factor for different products.

Table 3. The type of matrix containing ENP, release pattern and release factor for different products.

Product	Category	ENP Type	Type of Matrix Containing ENP	Release Pattern	Release Factor (%)	Reference
Sunscreen	Sunscreen	TiO2, ZnO	Suspended in liquid	Down the drain	90	Only 0.03% ENP can penetrate into skin after 24 h exposure [45] and 10% loss in the sea
Day cream	Skincare/Cosmetics	Pt	Suspended in liquid	Down the drain	95	Assumption: approximately 5% losses to solid waste and other solid phases.
Facial cleaning gel	Skincare/Cosmetics	Nano-hyaluronic acid	Suspended in liquid	–	0	Based on the assumption: present in form of small vesicles that get approximately 100% adsorbed on the skin and loose nano form.
Facial cleaning gel	Skincare/Cosmetics	MnO2	Suspended in liquid	Down the drain	100	Technical guide document on risk assessment, 2003 [46]
Day cream	Skincare/Cosmetics	Au, TiO2	Suspended in liquid	Down the drain	95	Assumption: approximately 5% loss to solid waste and other solid phases.
Mask cream	Skincare/Cosmetics	Pt	Suspended in liquid	Down the drain	95	Based on the assumption: approximately 5% loss to solid waste and other solid phases.
Day cream	Skincare/Cosmetics	Nano-capsules	Suspended in liquid	–	0	Nano-capsules were described to be adsorbed completely by Lademann et al., 2013 [47].
Eyeliner	Makeup/Cosmetics	Iron oxides	Suspended in solid	Down the drain	10	90% of the product retention on the facial cotton
Concealer	Makeup/Cosmetics	Al2O3	Suspended in liquid	Down the drain	30	70% retention on the facial cotton
Foundation powder	Makeup/Cosmetics	ZnO	Suspended in solid	Down the drain	30	70% retention on the facial cotton
Toothpaste	Health and fitness	All	Suspended in liquid	Down the drain	100	Technical guide document on risk assessment, 2003 [46]
Toothbrush	Health and fitness	Ag, Au	Surface bound	Solid waste	0	Permanent coating
Body lotion	Health and fitness	TiO2	Suspended in liquid	Down the drain	100	Technical guide document on risk assessment, 2003 [46]
Shower spray	Health and fitness	SiO2	Suspended in liquid	Down the drain	80	20% loss during the spray
Shower gel	Health and fitness	ZnO, TiO2	Suspended in liquid	Down the drain	100	Technical guide document on risk assessment, 2003 [46]
Shampoo	Shampoo/Health and fitness	TiO2	Suspended in liquid	Down the drain	100	Technical guide document on risk assessment, 2003 [46]
Badminton Rackets	Sporting goods	All	Surface bound	Unclassified	0	Disposal of the products as solid wastes
Tire	Automotive	SiO2, Carbon black	Suspended in solid	Runoff	70	Based on the assumption: 70% of the particles deposited on roadsides are washed off of roadsides and reach urban runoff during rainfall.
Car coating (& polish wax)	Automotive	Ceramic, TiO2	Suspended in liquid	Down the drain	100	Kojima et al., 2011 [48]
Beverages, instant noodles, seasoning, snacks, sweets	Food	TiO2, SiO2	Suspended in solid	Down the drain	60	Oral exposure is assumed to be the main route in terms of Food. SiO2 and TiO2 are assumed to not dissolve during passage through the gastrointestinal tract. However, according to UNEP 2011, 30% of food gets thrown away [49]. Long-term retention in the body is not clear, but assumed to be 10%.
Beverage	Food	ZnO	Suspended in solid	–(as ions)	0	Oral exposure is assumed to be the main route in terms of Food. ZnO is assumed to be transferred into ionic form during passage through the gastrointestinal tract.
Cleaning detergent	Home and garden	TiO2, Ag, Ti	Suspended in liquid	Down the drain	100	Technical guide document on risk assessment, 2003 [46]
Filter	Home and garden	Ag	Surface bound	Unclassified	0	Permanent coating
Socks	Clothing and footwear	Ag	Surface bound	Down the drain	100	Technical guide document on risk assessment, 2003 [46]
Hair curling machine	Household appliances/Electronics	Ag, Ti, and ceramic	Surface bound	–	0	Permanent coating
Washing machine	Household appliances/Electronics	Ag	Surface bound	Down the drain	100	Tiede et al., 2011 [50]

3.3. Estimation of ENP Reaching the Aquatic Environment

The amount of ENP reaching the aquatic environment was determined from the concentration of ENP in the products containing them, the daily release quantity of this product type to the aquatic environment, and the market share of the products containing ENP.

ENP concentrations in products. The concentration of ENP in different products was estimated from the literature and relevant regulatory standards. Table 4 shows ENP concentrations for different products. Their values were found to vary to some extent among different products of the same category, as shown in Figure 2 for TiO₂, SiO₂, ZnO, and Ag. The difference in ENP concentrations is attributed to the different function of ENP in different products.

Figure 2. Concentration ranges of nano-scale TiO₂, SiO₂, ZnO, and Ag in different consumer product categories (see also Table 4).

Table 4. Concentrations of nanomaterials in different consumer products.

Table 4. Concentrations of nanomaterials in different consumer products.

Category	ENP Type	ENP Concentration (% by Weight)		References
		Low	High
Sunscreen	TiO2	10	25	Weir et al., 2012 [42], EC regulation on Sunscreen [24]
Sunscreen	ZnO	10	25	Weir et al., 2012 [42], EC regulation on Sunscreen [24]
Day cream/Cosmetics	Pt	10	20	Purest colloids: http://www.purestcolloids.com/colloidal-skin-conditioners.php [51]
Day cream/Cosmetics	Nano-lipobelle EQ10	0.5	4	Müller et al., 2007 [52]
Lotion/Cosmetics	Nano hyaluronic acid	60	100	Based on the similar product information: http://www.ebay.com.au/bhp/hyaluronic-acid-serum [53]
Cream/Cosmetics	Nano-capsules	0.5	4	Müller et al., 2007 [52]
Cream/Cosmetics	Au	20	20	Taufikurohmah et al., 2011 [54]
Cream/Cosmetics	SiO2	6	15	Tiede et al., 2011 [55]
Facial cleaning gel/Cosmetics	MnO2	0.001	0.001	Weir et al., 2012 [42]
Facial cleaning gel/Cosmetics	TiO2	0.001	0.001	Weir et al., 2012 [42]
Foundation/Cosmetic	TiO2	0.5	4	Müller et al., 2007 [52]
Eyeliner/Cosmetic	Iron oxides	3	3	Tiede et al., 2011 [55]
Eyeliner/Cosmetic	Nano Collagen	0.5	4	Müller et al., 2007 [52]
Concealer/Cosmetic	Al2O3	3	3	Tiede et al., 2011 [55]
Foundation/Cosmetic	Alumino-silicate oxides	3	3	Tiede et al., 2011 [55]
Eye shadow/Cosmetic	Au	3	3	Tiede et al., 2011 [55]
Foundation/Cosmetic	ZnO	3	3	Tiede et al., 2011 [55]
Toothpaste/Health and fitness	TiO2	1	15	Weir et al., 2012 [42], Tiede et al., 2011 [55]
Toothpaste/Health and fitness	SiO2	1	15	Weir et al., 2012 [42], Tiede et al., 2011 [55]
Toothbrush/Health and fitness	Ag	0.001	0.1	Weir et al., 2012 [42], Tiede et al., 2011 [55]
Toothbrush/Health and fitness	Au	0.001	0.1	Weir et al., 2012 [42], Tiede et al., 2011 [55]
Body lotion/Health and fitness	TiO2	0.001	0.001	Weir et al., 2012 [42]
Shower spray/Health and fitness	SiO2	0.001	0.001	Weir et al., 2012 [42]
Shampoo/Health and fitness	TiO2	0.001	0.001	Weir et al., 2012 [42]
Tire/Automotive	SiO2	25	35	Wik et al., 2005 [56]
Tire/Automotive	Carbon black	25	35	Wik et al., 2005 [56]
Coating/Automotive	Ceramic	10	10	Boxall et al., 2007 [11]
Coating/Automotive	Nano glass	10	10	Boxall et al., 2007 [11]
Coating/Automotive	TiO2	5	5	Boxall et al., 2007 [11]
Beverage/Food	SiO2	0	<1.5	Health Canada: http://www.hc-sc.gc.ca/fn-an/securit/addit/list/1-anti-eng.php [57]
Beverage/Food	ZnO		<1.5	Health Canada: http://www.hc-sc.gc.ca/fn-an/securit/addit/list/1-anti-eng.php [57]
Beverage/Food	TiO2		<1.5	Health Canada: http://www.hc-sc.gc.ca/fn-an/securit/addit/list/1-anti-eng.php [57]
Instant noodle/Food	SiO2	1	1	Health Canada: http://www.hc-sc.gc.ca/fn-an/securit/addit/list/1-anti-eng.php [57]
Sweets/Food	TiO2	0.34	0.34	European food safety authority, 2004 [58].
Snack/Food	SiO2	1	1	Health Canada: http://www.hc-sc.gc.ca/fn-an/securit/addit/list/1-anti-eng.php [57]
Seasoning/Food	SiO2	1	1	Health Canada: http://www.hc-sc.gc.ca/fn-an/securit/addit/list/1-anti-eng.php [57]
Cleaning/Home and garden	TiO2	5	5	Boxall et al., 2007 [11]
Cleaning/Home and garden	Ag	0.001	0.1	Boxall et al., 2007 [11]
Cleaning/Home and garden	Ti	5	5	Boxall et al., 2007 [11]
Antimicrobial coating/Home and garden	TiO2	5	5	Boxall et al., 2007 [11]
Clothing and footwear	Ag	0.0028	0.076	Benn and Westerhoff, 2008 [44]
Household electrical appliances/Electronics	Ag	100	100	Tiede et al., 2011 [55]

In some product categories, several different types of ENP may be used. For example, some manufacturers only added either TiO₂ or ZnO (up to 25%) to their sunscreen, while others used both. To estimate the individual ENP concentrations, TiO₂ and ZnO were assumed to contribute equally to the total of 25% w/w contents. This approach was used for all categories, in which an article contained several types of ENP with similar functionality.

Daily release quantity. The usage quantity of ENP-containing consumer products was estimated from surveys of personal care products consumption in other countries. Biesterbos et al., (2013) [59] conducted a survey covering every province of the Netherlands. Both females and males were surveyed about usage patterns of personal care products in the past year. A wide range of personal care products were included, such as general hygiene, hair care, skin care, cosmetics (skincare products and makeup), and sunscreen. The collected data were used to determine the daily usage of these personal care products. While the study provided useful average usage data, no information was given about data variation such as standard deviation, range, or percentiles. Hall et al., (2007) [60] conducted a Europe-wide survey that focused on cosmetics but without breaking down the respondents by age. Loretz et al., (2008, 2006, and 2005) [50,61,62] did a survey to investigate the usage of cosmetics and personal care products over two weeks among females in the USA.

The literature review provided useful information but also indicated that the usage quantity for the same product can differ considerably between countries. For example, the per capita (pc) daily usage quantity range for body lotion was 0.4–21.4 (g/pc/d) in the Netherlands [59], 1.84–7.25 (g/pc/d) in Europa [60], and 3.8–13.8 (g/pc/d) in the USA [61]. The difference on the usage quantity among different countries may be attributed to variations in the survey, such as respondents’ age, ethnic group, body shape and gender, individual habits, and weather. To determine the usage quantity for food and beverages in Singapore, we selected data to be as representative as possible for Singapore, as detailed in Table 5. In addition to the abovementioned references, the Technical Guidance Document on Risk Assessment [46], a survey in the Exposure Factors Handbook [63], and the approach used in the UK to estimate nanomaterials’ release into water [55] were also used to collate data on the usage pattern of car coatings/polish, food consumption, cleaning products, and clothing. Table 5 summarizes the usage quantity for each article.

Table 5. Summary of daily per capita usage quantity for different products.

Table 5. Summary of daily per capita usage quantity for different products.

Product Name	Category	Usage Quantity (g/pc/d)
		Low	High	References
Sunscreen	Sunscreen	0.04	1.90	Biesterbos et al., 2013 [59]
Day cream	Skincare/Cosmetics	0.1	1.1	Biesterbos et al., 2013 [59]
Eye cream	Skincare/Cosmetics	0.005	0.055	Assumption: 1/20 of the day cream.
Toner	Skincare/Cosmetics	0.5	4.3	Biesterbos et al., 2013 [59]
Night cream	Skincare/Cosmetics	0.09	0.9	Biesterbos et al., 2013 [59]
Facial cleaning gel	Skincare/Cosmetics	0.5	4.3	Biesterbos et al., 2013 [59]
Cream mask	Skincare/Cosmetics	0.3	3.3	Assumption: 3 times more than day cream
Foundation	Makeup/Cosmetics	0.002	0.132	Loretz et al., 2006 [50]
Foundation powder	Makeup/Cosmetics	0.002	0.132	Loretz et al., 2006 [50]
Eyeliner	Makeup/Cosmetics	0.0002	0.0003	Biesterbos et al., 2013 [59]
Concealer	Makeup/Cosmetics	0.006	0.06	Assumption: 10% of the usage quantity of the foundation.
Eye shadow	Makeup/Cosmetics	0.006	0.06	Assumption: 10% of the usage quantity of the foundation.
Toothpaste	Toothpaste/Health and fitness	1.515	2.669	Hall et al., 2007 [60]
Shampoo	Health and fitness	3.79	21.91	Loretz et al., 2006 [50]
Body lotion	Health and fitness	1.836	7.25	Hall et al., 2007 [60]
Shower gel	Health and fitness	6	23	Loretz et al., 2006 [50]
Tire	Automotive	0.120 (unit: g/km)	0.120	Gehrig et al., 2005 [64]
Coating	Automotive	0.3	0.3	Tiede et al., 2011 [55]
Instant coffee	Beverage/Food	1.0	1.0	Derived from annual instant coffee consumption of 2000 tons in Singapore [65]
Instant noodles	Food	3.6	3.6	Seasoning powder (15 g) is 14% of an instant noodle package weight (110 g). Daily intake of grain products is 3.7 g/kg-day, cited from 2011 Exposure Factors Handbook of the USEPA [63]. The range of BMI for Asians is 17–35 kg/m2 (WHO expert consultation report, 2004) [66]. The average height for male and female in Singapore is 1.7 and 1.6 meters, respectively [67]. Therefore, the estimated average body weight for Asians is assumed to be 70 kg. The percentage of instant noodles in total grain products is assumed to be 10%. Therefore, daily intake of instant noodles is 14% × 259 × 10% = 3.6 g.
Seasoning powder	Food	0.032	0.032	Carlsen et al., 2011 [68]
Snacks (Chips)	Food	0.42	0.98	Daily intake of snacks for Asians is 0.1 (± 0.04) g/kg-day (USEPA, 2011, Exposure factors handbook) [63]. The estimated average body weight for Asians is assumed to be 70 kg. The percentage of chips in total snacks is assumed to be 10%. Daily intake of chips is = 7(± 2.8) × 10% = 0.7(± 0.28) g.
Sweets (Chocolate candy)	Food	0.14	0.14	Daily per capita intake of sweets is 0.4 g/kg-day in Exposure Factors Handbook [63]. The estimated average body weight for Asians is assumed to be 70 kg. The percentage of chocolate candy investigated in our inventory in total sweets is assumed to be 5%. Individual daily intake of chocolate candy is 0.4 × 70 × 5% = 0.14 g.
Cleaning	Home and garden	110	110	Technical guide document on risk assessment, 2003 [46]
Clothing	Clothing and footwear	4.45	4.45	Assumption: 1/20 of emission from clothing. Tiede et al., 2011 [55]
Washing machine	Household electrical appliances	1.375	1.375	Tiede et al., 2011 [55]

Market share. The market share for each product category was estimated from our market survey and from published data, as shown in Table 6.

Table 6. Estimated market share of ENP-containing consumer products in the Singapore retail market (“Market survey” refers to our assessments conducted in local stores).

Table 6. Estimated market share of ENP-containing consumer products in the Singapore retail market (“Market survey” refers to our assessments conducted in local stores).

Category	Market Share (%)	Reference
Sunscreen	85	Market survey: out of 27 products, 23 are likely to contain ENP.
Skincare/Cosmetics	1	Tiede et al., 2011 [55]
Makeup/Cosmetics	2	Tiede et al., 2011 [55]
Toothpaste/Health and fitness	78	Market survey: Out of 79 toothpaste, 62 are likely to use ENP.
Hair treatment/Health and fitness	3	Market survey: Out of 309 products 9 likely to contain ENP
Body lotion/Health and fitness	0.5	Tiede et al., 2011 [55]
Shower spray/Health and fitness	0.1	Tiede et al., 2011 [55]
Shower gel/Health and fitness	2.5	Market survey: Out of 162 products, 4 are likely to contain ENP.
Tire/Automotive	10	Tiede et al., 2011 [55]
Coating/Automotive	<1	Tiede et al., 2011 [55]
Beverages/Food	31	Market survey: Out of 148 products, 46 are likely to use ENP.
Instant noodles/Food	26	Market survey: Out of 232 products, 60 are likely to use ENP.
Seasoning/Food	0.1	Based on the assumption: market share is very low, 0.1%.
Snack/Food	21	Market survey: Out of 247 products, 53 are likely to use ENP.
Sweets/Food	13	Market survey: Out of 79 products, 10 are likely to use ENP.
Cleaning/Home and garden	<1	Tiede et al., 2011 [55]
Clothing and footwear	<1	Tiede et al., 2011 [55]
Household electrical appliances	<1	Tiede et al., 2011 [55]

Emission quantity of ENP. Data on ENP concentrations, release quantity, and market share were combined to estimate the emission quantity of each ENP using Equation 1. Table 7 shows the emission quantities of ENP reaching different environmental compartments (i.e., water, air, and soil). For the aquatic environment, municipal sewerage and urban runoff are the two main ENP-accepting bodies in Singapore. The most important ENP emitted into municipal sewage were, from high to low quantity: TiO₂, ZnO, SiO₂, Ag, and Au. For urban runoff, carbon black and SiO₂ were the major components that could reach the urban runoff collection systems (e.g., collection ponds, channels, and eventually reservoirs).

In addition to entering into the aquatic environment, ENP can also be transported to soil and air compartments during the use and disposal of the products (see Table 7 for the respective release factors). Solid waste is incinerated in Singapore and only a very small portion of ENP can pass through these modern incineration plants with state-of-the-art flue gas washing and effective filtration systems [69]. The ash residue in which any remaining ENP may be collected is then transported to a landfill on Semakau Island, thus they are removed from Singapore’s mainland. The aquatic environment is the main accepting body for the released ENP identified in this study.

The daily per capita release quantities obtained above can be multiplied by the number of people living in Singapore (5.3 million in 2013 [70]) or, in the case of automotive products, the number of registered vehicles (969,910 in 2012 [27]) to obtain the total quantity of ENP released annually into the aquatic environment of Singapore. The estimated ranges of release of the predominant ENP are: TiO₂ (73.5 to 969.0 t/y), followed by ZnO (1.2 to 272.0 t/y), SiO₂ (40.9 to 196.0 t/y thereof 21.2 to 29.7 t/y from road surface runoff), carbon black (21.2 to 29.7 t/y, mostly from runoff), Ag (26.7 to 27.5 t/y), and Au (0.002 to 2.600 t/y).

Table 7. Summary of quantity of ENP reaching different environmental compartments.

Table 7. Summary of quantity of ENP reaching different environmental compartments.

No.	ENP Type	Product/Category	Release into Water	Emission Quantity (g/pc/d)		Release Elsewhere	Emission Quantity (g/pc/d)
				Low	High		Low	High
1	TiO2	Sunscreen, automotive, cleaning, skincare, makeup, toothpaste, shampoo, body lotion, shower gel, beverages	Down the drain	3.80E–02	5.01E–01	Losses in the sea, solid waste	2.30E–04	2.50E–02
2	ZnO	Sunscreen, makeup, skincare	Down the drain	5.93E–04	1.41E–01	Solid waste	6.59E–05	1.56E–02
3	Pt	Skincare	Down the drain	7.20E–07	3.17E–04	Solid waste	3.79E–08	1.67E–05
4	Au	Skincare, makeup	Down the drain	1.21E–06	1.36E–03	Solid waste	2.25E–07	7.33E–05
5	MnO2	Skincare	Down the drain	1.52E–09	1.30E–08	N/A	N/A	N/A
6	SiO2	Skincare, toothpaste, shower spray, beverages, instant noodles, seasoning, snacks	Down the drain	1.02E–02	8.59E–02	Solid waste and aerosol in the air	4.12E–03	9.14E–03
7	Alumino–silicate oxides	Makeup	Down the drain	1.80E–08	1.19E–06	Solid waste	4.20E–08	2.77E–06
8	Iron oxides	Makeup	Down the drain	3.00E–08	9.00E–06	Solid waste	2.70E–07	8.10E–05
9	Al2O3	Makeup	Down the drain	2.40E–08	7.20E–06	Solid waste	5.60E–08	1.68E–05
10	Cu	Makeup	Down the drain	1.08E–08	1.08E–07	Solid waste	2.52E–08	2.52E–07
11	Si	Makeup	Down the drain	1.08E–08	1.08E–07	Solid waste	2.52E–08	2.52E–07
12	Collagen	Makeup	Down the drain	2.00E–10	2.40E–09	solid waste	1.80E–09	2.16E–08
13	Carbon black *	Tire	Runoff	5.98E–02	8.38E–02	Other environmental compartments: air and soil.	2.56E–02	3.59E–02
14	SiO2 *	Tire	Runoff	5.98E–02	8.38E–02	Other environmental compartments: air (airborne particles) and soil.	2.56E–02	3.59E–02
15	Ceramic	Car coating	Down the drain	1.20E–04	1.20E–04	N/A	N/A	N/A
16	Nano glass	Car coating	Down the drain	3.00E–05	3.00E–05	N/A	N/A	N/A
17	Ti	Home cleaning	Down the drain	5.50E–03	5.50E–03	N/A	N/A	N/A
18	Ag	Cleaning, clothing and footwear, household electrical appliances	Down the drain	1.38E–02	1.42E–02	N/A	N/A	N/A

* Unit: g/car/d.

4. Conclusions

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