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*
It is generally accepted that formation of diolepoxide-derived DNA adducts such as ones depicted in following [Figure1](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3812823/bin/nihms68761f2.jpg) is the principal metabolic activation pathway leading to cancer initiation in mammals (Courtesy of <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3812823/>). 

![Figure1](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3812823/bin/nihms68761f2.jpg)

Polycyclic aromatic hydrocarbons (PAHs) are a class of environmental contaminants ([Figure2](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3812823/bin/nihms68761f1.jpg)) that has long been suspected for genotoxicity, metabolism and metabolic activation, DNA adduct formation, mutagenesis, and carcinogenesis. Although, PAHs are relatively non-reactive chemicals toward biological macromolecules under physiological conditions, upon entering the body, our cellular defense system attempts to “remove” these foreign substances by metabolism, which is principally done in our liver, mainly catalyzed by the cytochrome P450 enzymes. Upon metabolism, most PAHs, with multiple hydroxyl groups, become more polar and water-soluble to be excreted out of the body. However, metabolism of some PAHs generates certain reactive intermediates (see benzo[a]pyrene; [Figure 1]) that are capable of forming damaging covalent adducts with nucleic acids (*e.g.*, free $\ce{DNA-NH2}$ groups), eventually leading to genotoxicity and cancer.

![Figure2](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3812823/bin/nihms68761f1.jpg)