No. First, when one speaks of allergies to nickel, these are skin allergies and not food allergies. Moreover, many commonly con- sumed foods (vegetables, cereals, cocoa, seafood, etc.) contain numerous traces of metallic elements, especially nickel (rhubarb, cocoa). The content of these elements is unchanged after cooking - even prolonged cooking - in stainless steel utensils. A comparison between the salting-out rate of utensils made of glass (inert product) and stainless steel shows that in fact stainless steel does not contribute to nickel enrichment of foods after cooking, the nickel content remaining identical to that of the raw foods.
Finally, other works show that slightly more extensive salting-out of nickel (and other metallic elements such as chromium or iron) is sometimes observed on new utensils; this phenomenon is re- lated to the existence of fine metallic particles coming from final polishing. Most manufacturers of household products made of stainless steel, moreover, recommend a first trial use with water brought to the boiling point, followed by cleaning. All these works have been summarised in the WHO dossier concerning nickel in food and drinking water.
Most of these allergies are caused by pure nickel, in massive form or in coating form (nickel plating). In stainless steels, nickel has by no means the same behaviour and is “trapped” in the alloy with no possibility of dissemination outside.
This inertia is related to the existence of a protective coating which forms spontaneously on stainless steels in contact with air; this coating is sufficiently tight to prevent the dissemination of metallic species in the ambient environment.
The stainless steel quality should, however, be compatible with use in the presence of sweat which is more or less acid de- pending on the person. Only those stainless steel grades that have been resulphurised (in order to improve their machinability) are potentially allergenic because of their mediocre corrosion resistance in acid sweat: they are no longer used in products intended for contact with the skin.
The nickel alloys and stainless steels used for the manufacture of prostheses are in widespread use and standardised in many countries throughout the world (Europe, USA, Japan). The world literature has identified less than 250 cases of allergies caused by nickel-base alloys in which thin steel orthodontic wires are also involved.
Most dental alloys containing nickel are of the nickel/chromium type and their corrosion resistance in the buccal environment is especially due to ... their chromium content; nickel/chromium dental alloys contain up to 70% nickel. During the corrosion process, however, it is not the nickel content that plays a decisive role for nickel release, but the quantity of chromium present in the alloy. In general:
- Either the alloy is strictly inert in the surrounding environment - in which case it is said to be “passive” - and none of its constituent elements is disseminated in the environment;
- Or there is corrosion of the alloy - in that case considered as unsuitable for the use made of it - and its constituent elements migrate into the environment in the same proportions as those encountered in the alloy in the solid state.
The salting-out of nickel (or any other metallic element) from an alloy is therefore intimately related to the alloy’s corrosion resistance in the working environment (sweat, saliva, etc.). In the absence of corrosion, there is no salting-out. The problem there- fore is not whether the alloy contains nickel but whether its intrinsic corrosion resistance is sufficient in the biological or chemical environment in question.
Alloys containing nickel - including stainless steels - have under- gone numerous experiments, in vitro and in vivo, within the framework of medical use, both on bone prostheses and on subcutaneous prostheses. Here again, the problem is not so much whether the alloy contains nickel but whether its corrosion resistance in a biological environment is suitable. Numerous scientific teams are looking to define optimised solu- tions, guaranteeing, through appropriate tests, their harmlessness vis-à-vis patients; in most cases, these solutions are covered by regulations and standards in most industrialised countries.
There are numerous standards governing the use of nickel alloys and stainless steels in applications relating to surgery and the dental art; these standards were worked out based on rigorous experiments in the laboratory and in the field. The processing and use of these materials is then based on compliance with strict Quality procedures.
Attempts to market bactericidal materials such as tissues have until now proved resounding failures. Concerning this subject, most hygiene specialists agree that the effects are not very selective and that there is much collateral damage, eradicating both pathogenic species and useful species.
The flood of patents for bactericidal stainless steels is apparent- ly for the time being not followed by major industrialisation; the European stainless steel producers have declared themselves opposed to the introduction in the Community market of such products whose purpose goes against the “healthy, hygienic” image of stainless steel.
It is indeed inconceivable combining:
- the qualities of inertia specific to materials designed for con- tact with food or for medical applications, and
- ... the concept of a “killer material” linked with the salting-out of chemical species which could eradicate certain micro-organisms dangerous for health.
Food and medical security is based above all on the application of and compliance with strict hygiene procedures (cleaning, disinfecting); in this area, stainless steels are perfectly well armed, given their remarkable corrosion resistance in the most diverse detergents and disinfectants.
No. In a stainless steel, chromium is exclusively in metallic form or in the form of “0 valency” chromium.
Chromium VI is one of the ionic forms of chromium (with chromium III) that is encountered in certain chemical, mineral or organic compounds (chromates) used in surface treatments and paints, for example; these compounds have physical, chemical and toxicological properties that have strictly nothing in common with those of the “metal” chromium forming part of the composition of stainless steels.
Due to their corrosion resistance, stainless steels are perfectly suitable for contact with food and, apart from the regulatory aspects mentioned further on, the steel grade will be chosen as a function of the food environment in question and the applicable hygiene protocols. This is especially true in the food processing industry, where the environments can be of highly variable corrosiveness (milk, mustard, cured meats, etc.) and where due to hygiene requirements, cleaning and disinfecting products that are likewise highly aggressive can be used (chlorinated alkaline detergents).
At the European level, there are for the time being no Community regulations concerning stainless steels intended for contact with food; a directive on the subject will be issued, in the best of cases, on the 2006 horizon.
Since 1976, France has had national regulations on the subject (order of 13 January 1976) specifying the authorised minimum and maximum content of stainless steel alloying elements (13 % chromium as a minimum, 4 % molybdenum and copper as a maximum).
French Standard NF A 36-711 (December 2001) now supplements this regulatory system by specifying the grades suitable for contact with food, under the conditions of acceptance of the French decree. These grades are also referred to in Europe- an Standard EN 10088 on “General-purpose stainless steels”. In Italy, stainless steels for food applications must have undergone successfully the migration tests stipulated in various environments (water, alcohol, olive oil and acetic acid); most stainless steels satisfy these tests.
In the USA, the FDA has since the 1960s classified stainless steel among GRAS (Generally Recognised As Safe) materials irrespective of the grade. Since July 2002, ANSI Standard NSF 51 specifies that series 200 and 300 stainless steels, and the cutlery grades, are accepted without further testing. For other series (400), a minimum resistance to a standardised salt spray test is required
It is possible to certify the compliance of a stainless steel with the requirements of French regulations (order of 13 January 1976). In this case it is recommended that the order be established as per French Standard NF A 36-711 which can ensure that the products supplied indeed comply with the requirements of the regulations in force.
The choice of stainless steel grade must of course be appropriate for the operating conditions: type of food environment, working temperature and cleaning conditions. The guaranteeof suitability for contact with food applies only if the stainless steel grade is appropriate - especially as regards its corrosion resistance - for the planned application.
No. Stainless steels are standardised and the corresponding standards define precisely the alloying elements - in kind and quantity - that it is possible to use in their manufacture (chromium, nickel, manganese, molybdenum, etc.). No heavy metal - such as lead, mercury or cadmium - forms part of the composition of stainless steels.
On the contrary, in many respects, the manufacture of stainless steels is an ecological business: almost 70 % of the raw materi- als used for their manufacture come from the recycling of end- of-life products or from joint products of processing (scrap, machining chips).
Increasingly efficient dust removal techniques have now reduced to a minimum emissions into the ambient air. Cooling systems in closed or semi-open circuit are becoming the rule, thus limiting the potential sources of water pollution. Finally, certain joint products can be used in public works as backfill materials, thus making it possible to limit operation - often with pollutant emissions - of new quarries.
Stainless steel can be recycled indefinitely by steel plants to again manufacture stainless steel, of identical quality.