Question

Which of the following is a contact poison?
A. Mercury tablets
B. DDT
C. Cryolite
D. Lead arsenate

Answer 1

Hint: As the name suggests, contact poisons are those chemicals or toxins which are absorbed in sufficient quantities by direct skin contact and lead to toxic effects and sometimes result in death in an individual.

Complete step by step answer:
The skin which is the main route for any entry of chemical substances. Two factors that affect the absorption are the concentration of chemical and surface area of the point of contact.

Let us discuss all options and its toxicities step by step:
Mercury tablets: Mercury which is a chemical element represented as Hg with atomic number 80. Mercury in its pure elemental form is not harmful to human beings but when it forms compounds then it is harmful. It plays an important role to keep insects away. Hence mercury tablets are not poisoned contact.

DDT: which is commonly known as dichlorodiphenyltrichloroethane with the chemical formula \[{{C}_{14}}{{H}_{9}}C{{l}_{5}}\]. It is colorless in color, tasteless in taste and odorless in odor and crystalline in a chemical compound. For DDT in which one book cited said that this chemical along with other pesticides caused cancer and for agricultural use, it is a threat to wildlife mainly for birds.

Cryolite: It is a chemical compound with the formula with a repeating unit. It is glassy, colorless to white, sometimes reddish, brownish, and rarely grey. In a molten state, cryolite used as a solvent for \[A{{l}_{2}}{{O}_{3}}\] (aluminium oxide) in the Hall-Heroult process which is used for refining aluminium. It is used as insecticides and pesticides. In studies using animal laboratories, on an acute basis cryolite generally is slightly non-toxic.

Lead arsenate: It is a chemical compound with the formula \[PbHAs{{O}_{4}}\]. Lead exists in its +2 oxidation state. It is white in solid and insoluble in water. Because of the presence of both lead and arsenic, it is highly toxic. It was widely used as an insecticide that had arsenic in it.
In the following chemical reaction, there is the formation of the desired product called lead arsenate as a solid precipitate.

\[Pb{{\left( N{{O}_{3}} \right)}_{2}}+{{H}_{3}}As{{O}_{4}}\to PbHAs{{O}_{4}}+2HN{{O}_{3}}\]

When there is an accumulation of nail, hair, and skin by chronic exposure it may result in Mess Lines.
Therefore, by observing the toxicokinetics (it is the study of the absorption, distribution, metabolism, and excretion of a xenobiotics) of given options it is concluded that the correct option is (B).
Let us see xenobiotics in brief: They are chemical substances that are found with an organism which are not naturally present with an organism.

Note: Both DDT and lead arsenate are highly toxic but most of the arsenic compounds are not completely absorbed by the skin and that is why it is likely to get into our body through our skin. Hence, it is concluded that DDT is considered as a contact poison. US EPA (United States Environmental Protection Agency) banned the use of lead arsenate on food crops in 1998.