According to the book Human Engineering guide for Equipment Designers, authored by Wesley E. Woodson and Donald W. Conover and published by the University of California Press, suggests the following effects at various levels of g-force for Transverse, positive and negative g-forces.
Effects of prolonged G
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Transverse G forces
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2 g - visual acuity is reduced
4 g - gross body movement is difficult
8 g - respiration becomes difficult
10 g - difficult to hold head up, can't move limbs
12 g - breathing difficult without mechanical help
14 g - vision begins to fail
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Positive G forces
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2 g - visual acuity is reduced
4 g - peripheral blindness, limb movement difficult
5 g - temporary blindness and loss of body control
6 g - unconsciousness
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Negative G forces
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2 g - diminished vision and head pains
3 g - conjunctival haemorrhage, red-out, and mental confusion
4 g - probably haemorrhage and retinal bleeding
With transverse g-forces being in the forwards directions from the chest (forcing a person into the back of their seat, positive being an upwards direction (driving the person into the floor) and negative being in a downwards direction (lifting a person off their seat).