3.4  The Mechanisms of the Chemical Burn During the Contact Between the Aggressor and the Eye

Finally, we could sum up the action of corrosives on biological material to three levels of interaction:

•Modification of the balances of chemical reactivity: acid–base or redox reactions

•Modification of biological molecules (by reactions of addition or substitution), for instance, by alkylating agents or as a consequence of a coagulation of proteins

•Disappearing of an active entity for cases such as chelating agents (for instance, the fluor ion) or solvents

3.4  The Mechanisms of the Chemical Burn During the Contact Between the Aggressor and the Eye

3.4.1  The Different Elementary

Types of Chemical Reactivity

The analysis of the various characteristic reagent functions of the irritant or corrosive molecules and the

nature of the biological constituents leads to the identification of a restricted number of reagent couples as mentioned in Sect. 3.2.2. In practice, there are six types of elementary reactions:

•Acid–base reaction

•Redox reaction

•Addition

•Substitution

•Chelating reaction

•Solvation

The acid–base couple is the most known because of the massive and ubiquitous use of the acids and bases in work environment as well as in homelife during chemical assaults. But it is not the only cause of ocular irritations or corrosions. An oxidizer or a reducing agent can also generate lesions to tissues.

What happens between both protagonists in this case? The oxidizer is going “to seek” from the reducer the transfer with definitive title of one or several electrons to complete its orbitals and so acquire a more stable external electronic cover. An oxidizer can answer a reducer and conversely (Fig. 3.55).

This sketch enables the understanding of the notion of exchange between the chemical aggressor and the

target biological constituents of the eye. According to the type of reactive function of the aggressor and thus the type of elementary reaction with the target molecules, different types of entities will be exchanged. This may involve electrons for a redox reaction, ions for an acid–base reaction or for a chelating reaction, atoms or molecules for additions or substitutions.

This notion of exchange appeals to the concept of acceptor and donor. The chemical aggressor and the target can alternately be either the donor or the acceptor.

As mentioned above, an acid or a base that does not express in the same way is thus totally disarmed with regard to an oxidizer or a reducer. Any chemical reactivity between them is thus totally impossible.

Nevertheless, some molecular structures may have several various elementary functions (Fig. 3.56).

For instance, hydrogen peroxide H2O2 is both acid and oxidizing.

Hydrofluoric acid is a particular illustration because it associates the elementary mechanism of the acid and the mechanism of a chelation causing a major toxic effect.

Chelation can be defined as the appropriation of mainly metallic atoms by a molecular entity, the size of which is often bigger. It is another example of

Irritants and corrosives are:

•Acids and bases

•Oxidizing and

•Reducing agents

•Other reactions

•Solvents

•Chelating agents

Targets in skin and eye are:

•Lipids

•Carbohydrates

•Proteins of structure (amino acids)

•Proteins of function (enzymes)

•Minerals salts (Ca, Mg,...)

­elementary chemical reactivity that may hurt the ­biological structures in contact.

HF is a partial dissociated acid (pK = 3.2). It releases 1,000 times less H+ ions in water than the same quantity of hydrochloric acid (pK = –2.2).

Therefore, the measurement of the pH alone is not enough to inform about the aggressiveness of this acid.

Nevertheless, the lesions provoked by HF are dreadful and may even endanger the vital prognosis of the individual because of the cardiac complications that it can engender in case of associated facial projection. Why? Simply because to the initial destruction of the structures of the eye by its acid potential (ion H+), is added the chelating and toxic action of the fluoride ion (F−) (Fig. 3.57). This action will develop gradually, in situ in the layers of the cornea, as the HF breaks up. This results in a deep damage with a necrotic character.

In this particular case, one F− ion chelates two Ca++ or Mg++ ions so disrupting the biochemical metabolisms until the occurrence of cellular death and the necrosis of tissues. It is a movement of physiological balance. This mechanism explains the first historic reflex to strengthen the contributions in ions Ca++ or Mg++ ions to answer the need of chelation of the fluoride ion. This resulted,

in the last past years, in the generalization of therapy protocols mainly using calcium gluconate.

All in all, the conditions of the reactivity of a chemical “aggressor” are bound to the intimate features of its constituting atoms. Molecules, whether they are irritants or corrosives, fill in not only specific conditions of “interlocutor” but also of sufficient energy level as we mentioned in Fig. 3.58.

Such a more mechanistic approach of the reactional context is called “reflexive toxicology” [2] because it requires second thought and reasoning. It is not only indispensable any more to learn “by heart” lists of effects without understanding why or how they work. It is a very effective solution, which enables an important economy of means and time.

3.4.2  Energy Dimension

of Chemical Burns

For a long time, chemists have had an extremely accurate knowledge not only of the type but also the energy intensity of chemical reactions.