The Laplace pressure is the pressure difference between the inside and the outside of a curved surface that forms the boundary between two fluid regions.
[1] The pressure difference is caused by the surface tension of the interface between liquid and gas, or between two immiscible liquids.
Although signs for these values vary, sign convention usually dictates positive curvature when convex and negative when concave.
Yet when the diameter is ~3 μm, the bubble has an extra atmosphere inside than outside.
When the bubble is only several hundred nanometers, the pressure inside can be several atmospheres.
One should bear in mind that the surface tension in the numerator can be much smaller in the presence of surfactants or contaminants.
The same calculation can be done for small oil droplets in water, where even in the presence of surfactants and a fairly low interfacial tension
= 5–10 mN/m, the pressure inside 100 nm diameter droplets can reach several atmospheres.