18 Hidden Causes of High Neutrophil Counts That Doctors Want You to Know

11. Smoking

Cigarette smoking has a profound effect on the immune system, and one of its lesser-known consequences is neutrophilia. Research has shown that chronic smokers tend to have higher white blood cell counts, including an excess of circulating neutrophils. This occurs because smoking introduces harmful chemicals into the lungs, triggering a constant low-level inflammatory response. Neutrophils, as part of the body's first line of defense, are continuously produced in greater numbers to combat the perceived threat. Over time, this persistent inflammation can contribute to chronic lung conditions such as chronic obstructive pulmonary disease (COPD) and increase the risk of cardiovascular disease. Interestingly, former smokers often experience a gradual decrease in neutrophil levels after quitting, as the body begins to recover from inflammation. The link between smoking and elevated neutrophil counts highlights yet another reason to consider quitting—beyond lung health, it directly impacts overall immune system function and long-term well-being.

12. High-Altitude Exposure

People who travel to or live at high altitudes may experience a condition known as altitude-induced neutrophilia. At higher elevations, oxygen levels decrease, which triggers a physiological response in the body to compensate. One of these responses includes an increase in red blood cell production to carry more oxygen. However, studies have also shown that white blood cell counts, particularly neutrophils, tend to rise in individuals exposed to high altitudes for extended periods. This is believed to be due to mild, sustained hypoxia (low oxygen levels), which places stress on the body and stimulates the bone marrow to produce more neutrophils. In some cases, this effect can persist for weeks or months after prolonged high-altitude exposure, especially in climbers, hikers, and residents of mountainous regions. While generally not harmful on its own, altitude-related neutrophilia is an intriguing example of how environmental factors can influence immune function.