Composition

Describing a general composition for tube glues is rather complicated, because numerous glues are contained in tubes. Instead, in Table 12 two types of glues with their composition and application are listed.

Table 12

The type of tube glue and its composition (Sources: Product information).

Use

A lot of glues are contained in (small) tubes. These glues are taken as one category since the way they are used is very similar. Hence, it is expected that the exposure is also similar. These glues are: universal glues (including hobby and household glues) and contact glue (Table 12). Keep in mind that these glues may also be contained in other kinds of containers and do not reflect the entire segment of universal glues, for example. In Table 12 an overview of the glues is shown with their components and trademarks.

The tubes vary in size and range from 5 ml to 125 ml glue per tube. The glues are liquids or gels with moderate viscosities. The glues are used for many purposes, but generally for small tasks, e.g. tinkering or gluing of photos. The glue is simply put on the surface to be attached. The glue is then spread (with tool, finger, or tip of the tube; not always necessary for smaller surfaces) to cover the surface and dried for a few minutes. According to the directions on the container, one should check whether the glue will attach properly using one’s finger. Then the other object is pushed onto the surface and the objects are tamped tightly to each other. For somewhat smaller surfaces or where strength is not so important, simply holding the surfaces together should be sufficient.

Contact glues often require two-sided application of the glue so that the adhesion is glue-glue. This increases the chance that an individual will be dermally exposed (assumed by twofold).

3.1.1. Scenario tube glue

Two small objects are glued together with 10 ml universal glue. According to an observational study about consumer use of DIY products (Magré 2005)^[6] the average duration for a glue job for small surfaces takes approximately 6.5 minutes. Although this figure also includes glues for other kinds of use, it is also considered indicative for the use of tube glues. Time spent on gluing is heavily dependent on the surface area to be treated and not on what kind of glue is used (except for super glue due to its very short working time). A default application duration of 10 minutes is taken into consideration. The time volatile compounds could evaporate may be longer because the glue needs to harden. Assuming that a person stays in the room after use, an exposure duration of 240 minutes will be used as default. In the model ‘evaporation from increasing area’, it is assumed that evaporation can occur during this time, which is a worst case assumption.

The amount of glue also depends on the surface to be treated. In the report by Magré, conclusions considering the product amount used per square metre were not drawn. According to the directions for use of the products per 100 ml, a surface of 0.2 m² can be treated. The amount of glue, set as default at 10 ml per 0.02 m², will, for this situation, be less, a product amount of 9 g is calculated and a product density is 0.9 g/cm³.

A room volume of 20 m³ and a ventilation rate of 0.6 h⁻¹ are used when the room where an individual is working is not specified (General Fact Sheet^[3]). Otherwise, the specifications of the room, if specified, can be used. It is estimated that individuals use tube glue on a once a week basis. Dermal exposure can occur when glues are spread out with the fingers or when excess glue is removed manually. The exposure duration will not take much longer than the application duration, because individuals will wash off glue immediately after use. In general, only the fingertips of one hand are used to spread out tube glue. One fingertip is approximately 1 cm^{2 [20]}; considering two fingertips provides a dermal exposure area of 2 cm². A small experiment with glue provided an average product amount of 0.06 g (maximum was 0.09 g) when glue was spread out with one finger (see Appendix A.2). The dermal load is therefore set at 0.08 g for tube glue.

Use

The bottles or jars contain glue in the range of 25 to 750 grams. In general the glues are used for larger surfaces than the tube glues. The most common known bottled glue is wood glue, which is used for various glue jobs from small to large. The glue is easily squeezed from the bottle onto the objects and spread out with one’s finger, tool or with the nozzle. To keep the glues free from fungi and bacteria, preservatives are added. In addition, additives may be present to make the glue water resistant. Hobby or universal glues also display a wide range of use and are used similarly to wood glues (see Table 13).

The exposure duration is not expected to be much longer than estimated for tube glues. One does not have to work precisely in this case and that saves time. Although bottled glues are taken up as one category their use and hence exposure may differ considerably. Therefore, two scenarios will be described with specific defaults for universal/wood glue and construction glue, respectively.

For PVC glue (for hard plastics) no separate scenario is described. To calculate inhalation exposure the default parameters for universal/wood glue can be used. PVC glue is applied with a brush, which is attached to the lid. Glue is applied to both ends of PVC pipes and then stuck together. Dermal exposure will be lower than is expected for other bottled glues and therefore considered negligible.

3.2.1. Scenario universal/wood glue

According to the report by Magré (2005)^[6] average application duration for small surfaces is 6.5 minutes. As a default scenario for gluing small surface areas during hobby use (e.g. for making a glider model) the application duration is set at 20 min (estimation) for wood glue and universal/hobby glue alike. It is expected that the individual will stay in that room for four hours in total. The applied amount varies per job and the surface to be treated. For the hobby task it was estimated that approximately 10 gram is needed (personal estimation). This relates to a surface area of 0.04 m² according to product information. It is assumed that this task is carried out in a small room (20 m³ room volume with a ventilation rate of 0.6 h⁻¹). The inhalatory exposure to bottled glues occurs from the (open) bottle itself and during and after application. When the glue is spread with one’s finger dermal exposure also occurs. Assuming that one uses only two fingertips of one hand, a dermal exposure area of 2 cm² will result. Because spills can occur on the skin or the glue is spread out, a dermal load of 0.06 g is taken into account (see Appendix A.2), thereby assuming that spills may mount up to 0.09 g. As default, the dermal load is set to 0.08 g. It is assumed that the glue will be immediately removed after the task is completed. The dermal exposure duration is therefore equal to the application duration.

3.2.2. Scenario construction glue

Construction glue is more often used for larger tasks than wood glue, here the construction of a closet is described. Construction glues are often water resistant and have filling properties which make them suitable for certain tasks. The way of application is different from the application of wood glue. It needs more precision, because removal of construction glue from surfaces is more difficult. This means that exposure is expected to be different from universal or wood glue. The frequency, however, of these kinds of tasks is much lower in comparison to hobby use, where a frequency of twice a year is assumed.

A surface area of 1 m² is assumed is to be treated as a starting point. To treat such a surface requires approximately 250 g glue according to product information. Assembling a closet takes four hours (=exposure duration) of which 30 minutes actual gluing. The task is conducted in a non-specified room, so the default (small) room size of 20 m³ and a ventilation rate of 0.6 h⁻¹ are assumed. Parts which have to be attached must be pressed with clamps or nailed together for at least four minutes.

Inhalatory exposure may occur during application and hardening of the glue, which occurs under atmospheric humidity (reaction with water in air). Diphenylmethane diisocyanate (MDI) in combination with polyol reacts directly with water in the air and forms Poly-MDI or PolyUrethane (PU). MDI itself is a non-volatile (vapour pressure < 0.005 Pa) at room temperature. Due to the low vapour pressure and the reaction with water in the air it seems unlikely that MDI vapour is released. On the other hand, other constituents or by-products may evaporate from the product. When no evaporation can indeed be expected, based on the constituents, inhalatory exposure will be negligible.

Dermal exposure occurs during application of the glue when spreading with one’s fingers or spills during clamping of materials. Holding the glued materials with one hand while tightening the clamp with the other hand can result in a potential exposure surface of one hand palm which equals to 215 cm². Appendix A.2 describes that when a hand palm is completely covered with glue, the amount of the glue is approximately 1 g. Because the glue is hard to remove and accompanied by product information that advises wearing gloves, it can be assumed that subjects will be precautious. A dermal load of 1 g would be too high, therefore 250 mg is assumed.

Composition

The main constituent of super glue is Cyanoacrylate (75-100%). Cyanoacrylate is the active compound in super glue. In addition, filling agents, plasticizers and small amounts of solvent (normally acetone) may be present in very small amounts.

Use

Super glues are used for very small surfaces that have to be glued very strong and fast. Super glues are contained in small tubes or dosage pens of 2-3 grams per product. Super glues are often contained in small syringes which make it easy to dose small amounts of glue. An example of the use of super glue is the repair of a mug which has lost its handle (ear). One droplet per end of the handle should be sufficient to reattach the handle to the mug. Simply put mug and handle together and hold them for a few seconds to two minutes maximum. After 24 hours the glue has reached full strength. Product information warns about the fast and strong adhesive properties (super glue adheres very strongly to skin), but it is not advised to wear gloves.

3.3.1. Scenario super glue

At maximum it will take five minutes to glue a handle to its mug. The report by Magré^[6] shows an application duration of almost 18 minutes, but mentioned that the subject had problems gluing the object. The application duration found by Magré is therefore not representative for the use of super glues. The total exposure duration is set at 240 minutes as default when a subject stays in the room after use. Per event it is expected that an individual will not use more than a few droplets. According to a letter from APA (Aromatic Product Association, 1998; as cited in^[23]) the amount used for this type of glue will be 0.5 g per event^[23]. As default parameters 0.5 gram glue per event will be taken; this will be sufficient to glue at least 2 cm² according to the product information. Inhalatory exposure will be low because the amount used is very low. Still, product information warns for inhalatory exposure because cyanoacrylate (if present; vapour pressure is 70 Pa) may cause irritation to the airways.

The model evaporation from constant area for assessing the exposure from super glues was chosen, because the release area is very small. Dermal exposure may occur, because objects can be very small and therefore make it easier to spill. The amount that is spilt from surplus glue will be very low, because the total amount used is already low. Half a droplet size is considered (25 mg) as default, along with a surface area of two fingers (2 cm²). However, individuals will try to avoid any dermal contact since the glue is hard to remove and stuck fingers are hard to separate.

Composition

Two-component glues are composed from two separated formulations which have to be mixed together prior to use. Their composition (Table 14) is different from all the other glues in that the glue consists of two components: i.e. a resin and a siccative (hardener). The chemical reaction between the two components provides the strength of the glue. Two-component glues are used for many specific purposes which require fast and strong binding. In general, two-component glues are resistant against water, cold, heat and chemicals.

Table 14

Composition of polyurethane based two-component glue and composition of epoxy based two-component glue (Source: product information).

Use

There are two kinds of two-component glues available: glue based on polyurethane (PU) and the other based on epoxy resins. PU based glues are well suited for binding wood, concrete, stone, ceramics and different kind of plastics. Epoxy based glues are more suitable for metals, pottery, porcelain, glass, ivory and plastics^[25]. However, the purposes of the two-component glues may overlap, because there is no rule on which two-component glue to use for a specific task.

Two-component glues are most often contained in duo-syringes or in tubes, but sometimes also in buckets or cans (for larger tasks). There are very small duo-syringes that make sure of the correct ratio between resin and siccative, which is used for optimal adhesion and is therefore easy to use. The duo-syringe contains approximately 60 grams of glue (both resin and siccative together). Besides syringes also separate tubes (of 100 ml each) are used, where the products are designed in such a manner that an identical length of the lines of glue will provide the optimum volume ratio for optimal adhesive power. The resin and siccative is then to be mixed together (a mixing cup and a spatula is provided) until a homogeneous colour or mass is obtained.

The mixture is manageable for approximately 15 minutes (polyurethane based) to 90 minutes (epoxy resin based) before the mixture will harden. It is therefore important that the resin and siccative are separated until usage. Before hardening, solvents such as acetone and MEK (methyl ethyl ketone) can be used to remove spilled glue. After hardening, the mixture can only be removed by mechanical force. Dermal contact is to be avoided and wearing of gloves is advised according to product information. However, accidental dermal contact to the mixture may still occur.

In general, two-component glues are used for moderate sized surfaces. Epoxy resin based two-component glues are longer manageable and may be more suitable for larger surfaces. On the other hand, some PU based two-component glues are used to glue parquet and thus a large surface size (see section 3.5).

The mixing and loading process and application (hence the exposure) are similar for both kinds of two-component glues. Furthermore, no clear distinction can be made in the product amount or treated surface size between the uses of both two-component glues. For these reasons one default scenario will be described for the use of two-component glue.

3.4.1. Scenario two-component glue

The use of two-component glues is not assumed to be very frequent. They are used for many tasks, but these tasks are mostly specific tasks that are not very frequent. Therefore a default of three times a year is assumed.

The scenario of gluing a large broken vase with two-component glue is described here. Assuming the vase has broken in half leads to a surface area of 500 cm² which is to be treated. For such a surface area approximately 20 g of glue is needed according to product information. In contrast, the amount of glue used for the gluing events observed by Magré^[6] was very low (< 0.5 g and 4 g). The surfaces that were treated were consequently small; the task consisted of gluing the ear of a mug and a broken dinner plate. No room was specified and thus the specifications of a non-specified room are considered. The parameters for a non-specified room are 20 m³ for room volume and 0.6 h⁻¹ for ventilation rate.

Composition

PVAc dispersion or one-component PU glue or

Two-component PU glue

Note: Some wood parquet glues are alcohol based

Use

These glues are used to glue wood parquet where the parquet is meant to lie on the surface (parquet is glued to the floor) or to float above it (individual parts are glued together, but not glued to the floor). Because application between the two parquet glues is different, the product (amount) used may differ significantly. Furthermore, for parquet glued to the floor there are also two-component glues, which need to be mixed and loaded before they can be used. The application of the two-component glue is regarded as being similar to parquet glued to the surface. The application of glue for floating parquet is very different, which results in a different exposure. For this reason two scenarios and defaults will be described.

3.5.1. Scenario parquet glued to surface

Gluing parquet onto the surface in the living room was selected as a default scenario for parquet gluing. According to General Fact Sheet^[3] a living room has a volume of 58 m³ with a surface area of 22 m² and a ventilation rate of 0.5 per hour. The endurance of such a floor is estimated to be 30 years, which is the lifetime of massive parquet floors. However, people tend to redecorate. Therefore a frequency of once in 8 years will be assumed. The parquet glue is spread out over a floor surface with a glue spatula, that is, the surface that can be covered within 10 minutes. With the glue still wet parquet is placed and firmly tampered or maybe mechanically attached (nails). These steps are repeated until the entire room is covered. The product information shows a consumption rate of 1 kg per every 1-2 m². Assuming a realistic worst case this provides 22 kg of parquet glue needed to perform the task. Note that in most cases, the task is performed by professionals, but this scenario refers to non-professional users.

3.5.2. Scenario floating parquet

The task of constructing a floating parquet floor is considered easier than constructing a parquet floor on the surface. Assuming the same indoor environment as above (subsection 3.5.1) for constructing floating parquet; it will take less time and far less glue to conduct the task. According to the Dutch Environmental Information Centre (Milieu Centraal) the durability of laminate or thin parquet is ten years at maximum and the durability of use for an average floor cover in the Netherlands is eight years^[28]. Nowadays, laminate that can be clicked together is more prominent than laminate which requires gluing. Considering a person who prefers parquet the frequency of laying parquet may be higher. A frequency of every four years for constructing floating parquets will be considered as default. The task is estimated to take 4 hours where the construction itself is performed with intervals. Hence the exposure duration and application duration are the same. According to the product information 1 kg is sufficient to cover 30 m². For an area of 22 m² it is assumed that 750 grams of glue is used. The glue is applied in the groove joints and tongue of the parquet elements. The glue is allowed to be absorbed for 2-3 minutes before the next element is placed and tampered with a rubber hammer. It is assumed that 1 m² is treated per interval; the effective release area is then 1 m². This interval is repeated 22 times to complete the task (see also subsection 3.5.1. under application). Surplus glue must be removed with a moist cloth. The container is a flask with a nozzle that indicates that the glue can be administered precisely to the groove joints. Spills can occur when pressing the parts together. The exposure area will be approximately 50% of one hand palm (110 cm²). The dermal load is estimated to be 0.5 g.

Composition

Adhesive agent:

Copolymer acrylate dispersions or Glue based on synthetic resin^[24]

Solvents that can be present: Ethylbenzene^[29], Cyclohexane^[30], Xylene^[24], Toluene^[24], Water^[24]

3.6.1. Scenario carpet glue

Carpet glue is used to keep floor covers in their place and prevent them from warping. Carpet glue can be used for several kinds of floor covers such as, carpets, carpet tiles, and PVC floor covers. Here, the laying of a carpet floor in the living room is considered. The durability of the carpet is heavily dependent on the sort of carpet and the quality. The average durability according to the Environmental Information Centre^[28] is eight years. Here, it is assumed that the frequency of laying a carpet is once every four years.

A room volume of 58 m³ and a surface area of 22 m² is selected as default in conformance with the General Fact Sheet^[3]. According to product information an average of 1 kg carpet glue per 2.5 m² is used for laying carpets, providing a total product amount of approximately 9 kg carpet glue. The glue is spread over the surface with a glue spatula. A manageable time for this specific glue is 40 minutes. An alternative description of exposure is required, because exposure to a very high product amount released from large release areas is not realistic. Instead, the release area is set equal to the surface area one can treat per interval. It is assumed that 4 m² per interval can be treated, which will take 15 minutes per effort. Assuming that the carpet is already cut to size and is only moved to fit; application duration is 75 minutes. The total exposure duration is set equal to the application duration. During this process individuals may be exposed via inhalation or the dermal route. The contact rate of the glue is 30 mg/min (default) (see section 2.6) with a release duration of 75 minutes. The surface area will be approximately 50% of one hand palm, resulting in 110 cm² contact area.

Composition

Paste: Styrene acrylate dispersion, preservatives

Powders: based on cement/mortar

Use

Tile glues are glues for wall and floor tiles that must be attached to all kinds of surfaces such as cement walls, concrete, wood, chipboards, other tiles etcetera. Tile glues can be used while covering floors in the living room, hall and kitchens or covering walls in, for instance, kitchens and bathrooms. Tile glues are available in paste form, which are ready to use, and as powder. In general, the pastes are used to attach tiles to absorbing surfaces, while the powders are used to attach to non-absorbing surfaces. The powder is diluted with water until the desired ratio for the job is obtained. Tile glue (paste or obtained paste) is applied and spread out with a glue spatula. Wet stains or spills can be removed with water; hardened stains or spills must be removed mechanically (see product information).

3.7.1. Scenario tile glue

The scenario selected for tile glue holds for two walls in a bathroom, with a default room volume of 10 m^{3 [3]} having a surface area of 10 m² (two bathroom walls of 2 m wide and 2.5 m high). In this case, the surface area will be considered to be the area treated with tile glue. Tile glues are available in pastes based on primarily styrene acrylate dispersions or as powders. Ventilation rates are generally high in bathrooms according to the General Fact Sheet, where a default of 2 h⁻¹ was derived. The glue is applied to the tile or directly to the surface with a glue spatula. The tiles are then attached by pressing them while shifting the tile in the right position. This must be done within a 15 minute time frame in which the glue is still manageable. The use of glue is on average 1-2 kg/m². Assuming an average use of 1.5 kg/m², this yields a total amount of 15 kg tile glue to be used on 10 m². The task is considered to take 6 hours. Covering walls or floors with tiles is not expected to occur at high frequency (a frequency of once every two years is assumed).

Composition

Poly Anionic Cellulose (PAC) or methyl cellulose or Polyvinyl Acetate (PVAc) Preservatives^[25]

Use

Wall paper glues are available in powder form, which have to be suspended with water before use. The powder may contain PAC, methyl cellulose or PVAc and does not normally contain any solvents. The adhesive powder is poured into cold water in a proper ratio (according to use directions) and stirred until there is a solution without any lumps. According to product information 125 grams of glue powder can be mixed in 6 to 10 l water (this is for normal wall paper); there are also special and heavy wall papers, for which alternative wall paper glue is used. Although heavy wall paper glue is based on the same constituents, it is likely to have other volume percentages. The obtained suspension can be used for surfaces ranging from 30-50 m², depending on the type of wall paper. After the powder is in suspension the mixture can be easily applied to the wall paper with a brush. The wall paper covered with glue is folded to make sure it is properly soaked with glue and left to rest before attaching wall paper to the wall.

3.8.1. Scenario wall paper glue

In the default scenario the walls in a living room will be covered with normal wall paper. According to the General Fact Sheet^[3] the volume and surface area are 58 m³ and 22 m², respectively. A total surface area of 40 m² for the walls (corrected for doors and windows) is assumed for hanging wall paper. Applying wall papers is not a frequent DIY task but is assumed as a default once every two years.

Composition

EVA (Ethylene vinyl acetate; a thermoplastic) or other thermoplastic

2% Methylenediphenyl diisocyanate (MDI)^[27]

Use

Hot melt adhesives are solidified glues that are primarily for hobby use. They have a wide range of applicability and are used mainly for small surfaces such as connections of model parts and flowers for decorative purposes. The adhesive agent is a thermoplastic, such as EVA. A glue gun is required to use a hot melt adhesive. The glue gun is an electric device which heats up the solidified glue bar (65 g^[27]). The glue is inserted in the rear and then heated up for approximately five minutes. The temperature at which the glue is manageable is 150 to 180 degrees Celsius. As a result the glue bar will melt inside the glue gun. By triggering the glue gun, the melted glue is squeezed out onto the surface. The connection must then be realized within 15-20 seconds; otherwise the glue has cooled down too much. During the heating process MDI vapours may be released from the MDI containing glue bars. The vapours coming from the heating process can enter the breathing zone of the user^[27]. Although the volume percentage of MDI is rather low, it may still elicit irritant effects in the respiratory tract.

3.9.1. Scenario hot melt adhesives

The scenario of gluing the soles of the shoes (surface area approximately 100 cm²) with a hot melt adhesive is described here. The effective release area is, however, much lower due the gradual release of the hot melt in combination with the cooling down of the glue. The effective release area is estimated to be approximately 5 cm². During this task, one glue bar will be consumed (65 g). The task is conducted in a non-specified room. This room is therefore defined to have a volume of 20 m³ and a ventilation rate of 0.6 h^{−1 [3]}. In total, an application duration of 25 minutes will be taken into account which is spread over a longer period of time (gluing occurs with intervals). The default exposure duration is also set at 25 minutes due to a fast cooling down of the glue. The frequency of use is estimated to be once per month.

Exposure to a hot melt adhesive takes place during the heating of the adhesive. Inhalation exposure may be caused by a chemical reaction due to heating, or from evaporation at high temperature. ConsExpo does not include models to evaluate chemical reactions. An estimate of the amount of chemical produced in the reaction has to be made separately. The exposure can then be calculated using the instantaneous release model. Similarly, the evaporation models in ConsExpo do not include an option where the evaporation takes place at a different (much higher, in this case) temperature than the surrounding space (room temperature). As the evaporation in this case is likely to be much higher than evaporation at room temperature, the use of the ConsExpo evaporation model could lead to an underestimation of the exposure. Therefore it is advised to use the instantaneous release model in this case as well. The modelled concentration can be limited to the vapour pressure of the substance at room temperature to prevent the occurrence of unrealistically high concentration levels that may arise from the immediate release of the substance. Ticking the box: ‘limit the air concentration to the vapour pressure of pure substance’ in ConsExpo is required.

Inhalatory exposure takes place when the glue is being heated or melted. Vapours of MDI or EVA can reach the breathing zone of the user. Dermal exposure during use is considered negligible, because one can work very precisely with a glue gun and, moreover, the glue is too hot to spread over the surface. Before use, there is dermal contact when the glue bar is inserted. Assuming that only one hand will be used to insert the glue bar; 20% of a hand palm will be exposed. This equals a contact surface of 43 cm². The dermal load will be low, because the glue is still solid at room temperature. The glue is more or less rubbed off the glue bar. The dermal load is estimated to be in the order of 100 mg. In the EU RAR on MDI^[27] the dermal load from MDI alone was estimated to be 168 mg, which was a worst case estimation.

Composition

In contact spray the active component is SBR (Styrene-butadiene rubber). In the Netherlands a mixture of propane and butane (50%: 50%) is often used as propellant. A spray can contain about 30-35% of this mixture. According to product information the content of solid materials is 16%.

Use

Besides having glues in tubes, bottles, cartridges, syringes and canisters, there is also glue is also available in the form of spray. Generally glue spray is used on materials such as paper, cardboard, photos, textiles, cork and metal foils. Glue spray is also suitable for temporary attachment of materials. The glue is applied on both surfaces which are to be attached, except when porous materials are used, in those cases only one side should be sprayed with glue.

3.10.1. Scenario glue spray

The scenario of gluing a poster to a wall or door, or put in a frame is described here. This task is not performed often and thus a frequency of once per month is assumed. The surface of the poster is assumed to be 1.5 m². According to product information 300 ml (255 g; product density is 0.85 g/cm³) is required to glue the poster. Using the mass generation rate from subsection 2.3.3 (1.5 g/sec) provides a spraying duration of 170 seconds. The exposure duration is the time that a person stays in the room during and after the task. The total exposure duration is set at 240 minutes. The volume percentage of non-volatiles is 16% according to product information.

The room is not specified, therefore default values for room volume, room height and ventilation rate from General Fact Sheet for a non-specified room are considered in this case. These values are 20 m³, 2.5 m, and 0.6 h⁻¹, respectively. Dermal exposure can be estimated with the constant rate model. The exposed area will be half of both hands (430 cm²; estimate).