DamageVolumePlayer.cs

using UnityEngine;
using UniRx;
using System;
using System.Linq;

public class DamageVolumePlayer : MonoBehaviour {

    // Health should live in a different component. Just here for simplicity
    public FloatReactiveProperty Health = new FloatReactiveProperty(100);
    public float DamagePerSecond = 1.0f;

    public BoolReactiveProperty InDamageVolume = new BoolReactiveProperty(false);
    public BoolReactiveProperty InDamageNegateVolume = new BoolReactiveProperty(false);

    public BoolReactiveProperty ShouldTakeDamage = new BoolReactiveProperty(false);

    // The Damage Volumes will add/remove themselves from this collection
    public ReactiveCollection<DamageVolume> CurrentDamageVolumes = new ReactiveCollection<DamageVolume>();

    void Start () {

        // Check if in a damage volume
        CurrentDamageVolumes.ObserveCountChanged()
            .Select(_ => CurrentDamageVolumes.Where(v => v.IsNegateVolume == false).Count())
            .Select(count => count > 0)
            .DistinctUntilChanged()
            .Subscribe(inDamageVolume =>
               InDamageVolume.Value = inDamageVolume
            ).AddTo(this);

        // Check if in a damage negate volume
        CurrentDamageVolumes.ObserveCountChanged()
            .Select(_ => CurrentDamageVolumes.Where(v => v.IsNegateVolume == true).Count())
            .Select(count => count > 0)
            .DistinctUntilChanged()
            .Subscribe(inDamageNegateVolume =>
               InDamageNegateVolume.Value = inDamageNegateVolume
            ).AddTo(this);

        // Check if the player should take damage
        Observable.CombineLatest(InDamageVolume, InDamageNegateVolume,
            ( inDamageVolume, inDamageNegateVolume ) => inDamageVolume == true && inDamageNegateVolume == false)
            .DistinctUntilChanged()
            .Subscribe(shouldTakeDamage =>
               ShouldTakeDamage.Value = shouldTakeDamage
            ).AddTo(this);

        // Remove Health if the player should take damage every second
        Observable.Interval(TimeSpan.FromSeconds(1))
            .Where(_ => ShouldTakeDamage.Value == true)
            .Subscribe(_ =>
               Health.Value -= DamagePerSecond
            ).AddTo(this);
    }
}