Question: I have heard that benzene is a dangerous chemical for humans; it causes cancer and so it's use is restricted/regulated. But many organic compounds that we use contain benzene in some form such as oils, foods, even medicines (paracetomol). So is benzene really harmful to people's health?

Answer: 
First let us make clear that the products you mentioned do not contain benzene. Likely you intended that plenty of products are aromatic compounds and, as such, contain at least a phenyl ring (or contain at least a benzene-like motif, for those less familiar with chemical nomenclature).

After reading the following, that gives some anecdotal notes as well, it will be clear that the presence of a specific part of a molecule does not necessarily dictates its properties (toxicological in the specific discussion) to the whole. 

Translated from my old university notes:

> 
> .... many aromatic compounds are carcinogenic. After the London fire,
>  a law imposing the maintenance of chimneys was promulgated. As a
>  result it was noted that sweeper kids were at high risk of developing
>  scrotum cancer. This was lately connected to benzo[a]pyrene present in
>  the soot, and clearly established in the 1900s.
> 
> 
> Benzo[c]anthracene is carcinogenic as well, and *benzene* is among the
>  most powerful agents inducing leukemia. 
> 
> 
> In general strong carcinogens are strong electrophiles, such as
>  alkylants. On the contrary, benzene is electron rich. 
> 
> 
> Again in England, around 1960, a kind of plague was causing the mass
>  death of turkeys due to leaver cancer. It was established that as due
>  to to a toxin present in the feeds that, once metabolized, gave
>  epoxides as strong electrophiles.
> 
> 
> A link to aromatic carcinogenic properties was then made and
>  established. Rather than aromatic themselves, the real carcinogens are
>  their epoxides formed during metabolism. They act as substrate for the
>  nucleophilic attack by DNA bases as we will see in......
> 
> 
> 

Anything altering the stability or geometry of the metabolite epoxides will therefore modulate their reacivity and in turn the hazard posed by that specific aromatic compound.

This is also the case for [PAH](https://en.m.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbon) were, at least, a particular structure (*bay*) is linked to strong carcinogenicity. 

Edit. As this answer received a comment pointing to the fact that not all aromatics are carcinogenic I invite you to consider that the above

* *many aromatics* does not mean that all aromatic compounds are carcinogenic
* *anything altering....* means that each molecule must be considered as a whole. Functionalisation of benzene by a methyl group leads to toluene, for instance. The latter does not qualify as carcinogenic and should replace benzene whenever possible
* PAH is of course a subset of arenes. Among PAHs, again, not all compounds are carcinogens. However, in this case, a specific feature regarding the shape rather generally point to carcinogenicity (see link).

*yes benzene is very harmful to human health*
Benzene is toxic to humans, it can be oxidised by enzymes in the body (might be a P450 enzyme) to form benzene oxide which is an epoxide. This is 7-oxabicyclo[4.1.0]hepta-2,4-diene which can then react with DNA or other molecules in the body. If the epoxide reacts with water to form cyclohexa-3,5-diene-1,2-diol. Then this can be oxidised again to form 7-oxabicyclo[4.1.0]hept-4-ene-2,3-diol which can react with the DNA.

As a result of this DNA damage benzene is able to cause cancer, it is important to keep in mind that many carcinogens are in fact precursors of the carcinogen. They require some metabolic activation to be able to cause cancer.

Another example is hydrazine which is converted in the liver by formaldehyde and oxidation into diazomethane. As a result the liver is damaged.