The heat index (see chart below) is the "feels like", or apparent, temperature. As relative humidity increases, the air seems warmer than it actually is because the body is less able to cool itself via evaporation of perspiration.
 
As the heat index rises, so do health risks. When the heat index is 90°-105°F, heat exhaustion is possible. When it is above 105°F, it is probable. Heatstroke is possible when the heat index is above 105°F, and very likely when it is 130°F and above. Physical activity and prolonged exposure to the heat increase the risks.
 
Heat exhaustion occurs when the body is dehydrated.

• Symptoms: headache, nausea, dizziness, cool and clammy skin, pale face, cramps, weakness, profuse perspiration
• First aid: move to a cooler spot, drink water with a small amount of salt added (one teaspoon per quart)
• It can lead to collapse and heatstroke.

Heatstroke occurs when perspiration cannot occur and the body overheats.

• Symptoms: headache, nausea, face flushed, hot and dry skin, no perspiration, body temperature over 101°F, chills, rapid pulse
• First aid: cool person immediately, move to shade or indoors, wrap in a cool, wet sheet, get medical assistance
• It can lead to confusion, coma, and death.

The following chart shows the health risks as temperature and relative humidity rise.
 
 

The Heat Index
Air Temp (°F)	Relative Humidity (percentage)
	0	5	10	15	20	25	30	35	40	45	50	55	60	65	70	75	80	85	90	95	100
135°	120	126
130°	117	122	131
125°	111	116	123	131	141
120°	107	111	116	123	130	139	148
115°	105	107	111	115	120	127	135	143	151
110°	99	102	105	108	112	117	123	130	137	143	150
105°	95	97	100	102	105	109	113	118	123	129	135	142	149
100°	91	93	95	97	99	101	104	107	110	115	120	126	132	138	144	150
95°	87	88	90	91	93	94	96	98	101	104	107	110	114	119	124	130	136	140	150
90°	83	84	85	86	87	88	90	91	93	95	96	98	100	102	106	109	113	117	122	126	131
85°	78	79	80	81	82	83	84	85	86	87	88	89	90	91	93	95	97	99	102	105	108
80°	73	74	75	76	77	77	78	79	79	80	81	81	82	83	84	85	86	87	88	89	90
75°	69	69	70	71	72	72	73	73	74	74	75	75	76	76	77	77	78	78	79	79	80
70°	64	64	65	65	66	66	67	67	68	68	69	69	70	70	70	70	71	71	71	71	72

 

	=	Heatstroke risk extremely high			=	Heat exhaustion possible
	=	Heat exhaustion likely, heatstroke possible			=	Fatigue possible