SM, 118, "Pre-existing wards, mana barriers, active foci, sustained spells, and quickened/anchored spells are similarly affected. Reduce their Force by the absolute value of the background count. If the Force is reduced to zero or less, wards and mana barriers will collapse, foci will deactivate, and spells will fizzle. The enchantment on a quickened/anchored spell or ward/mana barrier will repair itself once removed from the background count, returning to its regular Force. Background count also affects astral visibility (see p. 114), which affects both assensing and astral combat."
As far as dual natured characters.... SM, 120, "Dual-natured creatures and spirits are typically uncomfortable within a mana ebb, even though it doesn’t cause them any harm directly." Note, Ebbs are from -1 to -6. Beyond that it becomes a Mana Void, SM, 120, "If a character exposes himself to the astral plane (either by astrally perceiving or projecting) while in a void, he risks having his astral form torn apart as it dissipates into the surrounding emptiness—as would air in a vacuum. Each Combat Turn the character is astrally active, he suffers Physical damage with a Damage Value equal to the absolute value of the void’s rating (so a –8 Rating void deals a DV of

. Dual-natured or astrally-perceiving characters resist this damage with their Body attribute, while pure astral forms (such as projecting characters) resist with their Willpower attribute. If the character has any form of astral armor (such as the Mystic Armor critter power), it applies to this test."