Thank you @sarahsweets and @~Christina for your posts.
@~Christina I am sorry your 29 year old stepon is so into vaping that I strongly advise him to seek medical advice in case he already has a breathing problem: such as asthma, or if his doctor knows of any other medical conditions that could be exacerbated by his excessive vaping.

However, today on BBC News, A ban on the sale of flavoured e-cigarettes has been announced by US President Donald Trump, after a number of vaping-related deaths.

Back to the UK.
UK scientists found the vapour could lead to changes in the lung's immune cells. See BBC News:
E-cigarettes: How safe are they?

E-cigarettes: How safe are they? - BBC News

In the UK, warnings and precautions about using e-liquids containing nicotine have already been published and enclosed in all nictonie-based e-liquids on sale:

* People should not vape if they are a non-smoker.

* Young people under the age of 18 years.

* Pregnant or breast-feeding women, as nicotine is known to cross the placenta and be excreted into breast milk.

* Those experiencing general ill-health, such as heart disease, stomach and duodenal ulcers, liver or kidney problems, long term throat disease or difficulty breathing due to bronchitis, chronic obstructive pulmonary disease (COPD), emphysema or asthma.
* Those who have an overactive thyroid gland or have a pheochromocytoma (which is a tumour of the adrenal gland that can affect high blood pressure).
* Those who have diabetes, because may affect their blood sugar levels.
* Those who are taking the following medicines:
* Theophylline (to treat breathing problems such as COPD
* Ropinirole (to treat restless leg syndrome)
* Clozapine, sold under the brand name Clozaril among others (to treat schizophrenia)

There are two methods of vaping.

1. Inhaling from the mouthpiece straight to one's lungs.
2. Inhaling to mouth and then to lungs.

I wonder if No. 1 is more likely to produce medical problems because the vapour only partly mixes with air when the vaping unitis used. Vapour would be more highly concentrated, therefore likely to cause ill heath when excessively used. As to whether e-liquids' fruit flavours are synthesized from pure fruit extracts, or, are these flavours EU Approved additives and E numbers, I don't know. In the UK there are an astounding 1,400 e-liquid flavours to choose from. VapeUK state all 10ml e-liquids are 'TPD compliant', or Tobacco Products Directive Compliant. Maybe this is why the UK seems to have a lower ill health rate from vaping, unless vapers are actively use their devices more than average. But maybe the US tobacco companies have a vested interest in producing tobacco-flavoured e-liquid, I don't know.

What I do know is I ceased smoking by occasional use of vaping, and no longer have need of vaping.