Technically it makes no difference if the air is extracted upwards, sidewards or downwards. I'd have a lot of fun with any Inspector that tried to convince me otherwise
However, air speed is important. As a rule of thumb this needs to be sufficient to capture both visible and invisible dust (actually the most dangerous) that occurs when using an e-file (the dust flies of at high speed and has to be sucked into the filter or ventilation system). However, if the air speed is too high then a) the noise increases and b) the carbon filter make not have enough time to capture the chemicals.
Cheap filters are often not capable of capturing the invisible dust. This dust will then hang in the air and if the salon is not ventilated can hang for weeks and increase in density like pollution.
The filter should also be capable of capturing any chemicals in the air. These are neutralised using an active carbon filter (like charcoal), so that clean air is then extracted back into the room. Many filters don't have these carbon filters and so don't protect against chemicals. Many of the filters on the maker that do have a carbon filter have such small filters that they are either useless or need changing very often.
We use a Swedish filter system that has 5.8kg of active carbon and this lasts for up to 400 customers before it needs replacing. It's also been independently tested to remove 99.97% of all dust and chemicals - no other filter system I know of has been independently tested.
Some of the poor quality filter systems I've seen on the market have a combined visible dust, invisible dust and active carbon filter that weighs less than 200grams. I fail to see how they can work effectively for more than a small number of customers.
Good filter systems are not cheap - approximately £1000. Although the unit we use can accept a second pipe and be used simultaneously on a second table (the cost is then £600 per table). But it's important to protect your health and a good investment. (Seat belts and airbags increase the cost of a car, but everyone accepts that safety cost).