The Formaldehyde Myth
Myth 1 - e-cigarettes produce formaldehyde at dangerous levels. The truth is crucially different. Only certain e-cigarettes MAY produce formaldehyde and under certain conditions. Most will not.
Myth 2 - the formaldehyde seen exclusively in exceptional circumstances represents a cancer risk similar to, or higher than burning tobacco. The truth is that formaldehyde is not the main toxin in cigarette smoke. It is naturally present and only over a certain threshold does it have (a weak) carcinogenic effect
Myth 3 - that you could be inhaling formaldehyde and not know it. You absolutely will not be - formaldehyde at levels well below the amounts found in the study on heavily used high-powered top-coil atomisers will produce a pungent and unpleasant smell and you are very unlikely to be able to vape this. You will certainly tell when it is occurring.
The power going through the ecig atomiser is a big factor in determining if there could be formaldehyde being generated. But this doesn't mean it is the only one. The factor that really matters is the temperature of the atomiser, and this can vary depending on how it is designed and used, so it isn't even true that all high powered devices will get hot enough to generate formaldehyde, and it is very unlikely in bottom-fed tank systems. You need to make sure they are however kept topped up so the wick is fed.
A good rebuttal in the press can be read here