Vent in wall

Making a hole in an external wall can weaken it, but in most cases the effect is so small that it does not compromise the wall's structural integrity. The key reasons are:

The hole is small.
A typical extractor fan or air vent is 100–150 mm (4–6 inches) in diameter.
Compared with the total wall area, very little material is removed.
Loads are redistributed.
Masonry walls spread loads around small openings.
A small circular hole does not significantly interrupt the load path.
Building standards account for this.
Walls are designed with a safety margin.
Small penetrations for vents, pipes, and cables are common and generally acceptable.
When it can become a problem

A hole may weaken the wall if:

it is very large (for example, creating a new window or door opening);
several holes are drilled close together;
it is cut near a corner, lintel, or other critical structural area;
it damages reinforcement in reinforced concrete or important structural elements.

Large openings often require a lintel or other structural support because they interrupt the wall's ability to carry loads.

Making a hole for a standard wall vent does not normally weaken the structure of the wall. The hole is small and is usually drilled between bricks or blocks without affecting the wall's load-bearing strength. A professional will also avoid cutting through structural supports such as lintels or beams. Once the vent is fitted and sealed correctly, the wall remains structurally sound while allowing proper ventilation. If a larger opening is required, it should always be assessed first to ensure the wall is adequately supported.

It's a tiny fraction of the walls load bearing capacity.

The wall carries loads through a much larger area

As long as it’s a small hole it doesn’t affect the structure if it’s a big hole we would need to study the area first before making any decisions.

modern buildings distribute weight effectively, and the bricks are designed to carry much more weight than the small section of material being removed

When a vent is fitted through an outside wall, it does not normally weaken the wall because the hole is small and is cut in a controlled way. Most external walls can safely take a properly sized core hole for a vent, as long as it is positioned away from important structural areas such as lintels, beams, wall ties, corners and major load-bearing points. The vent sleeve also helps keep the opening neat and protected.

Before making the hole, I would check what type of wall it is, where the services are, and whether there are any signs of cracking or weakness. I would use the correct size core drill, avoid forcing the opening, and make sure the vent is sealed properly afterwards to prevent water getting into the wall. If there was any doubt about the structure, I would get further advice before drilling.

Because the hole is small and avoids the main structural parts.
In most walls:
The weight is carried by studs (timber) or spread across many bricks
A vent hole is usually cut between those supports, not through them
So the structure still has enough strength to hold everything up.
If the hole were big or cut through key supports, then it would weaken the wall.

vent do slightly reduce the strength it's just not significant enough to notice it.
Brick/block walls don’t carry weight straight down The load spreads out.
So when you cut a small vent in, the wall around it redirects the load.

A vent hole slightly weakens a wall, but if it’s small and placed correctly, the wall redistributes the load around the opening so the structure stays strong.

A small vent hole doesn’t weaken a wall because of load redistribution.

In masonry, weight naturally "arches" around small openings, transferring the vertical load to the solid bricks on either side. Because standard vents (like for fans or dryers) are small relative to the total surface area, the wall's built-in factor of safety easily compensates for the minor loss of material without compromising stability.

it depends where you have made it and what is the load bearing in that area, however if it is a small hole then the bricks above are tied into each other but over time you will know if the wall structure is weakened if cracks appear near the vent hole. Best to put a concrete lintel above the hole to keep the load spread or dependant on whole size a catnic lintel.

"When making a hole for a ventilation grille in an exterior wall, it is verified that it is done in non-structural areas and that safe boundaries are respected. In this way, the structure is not weakened, as the loads remain distributed and the area is reinforced if needed."