Carbon monoxide (CO) is a colorless and odorless gas that can enter the human body through environmental exposure and especially tobacco use. CO binds to hemoglobin with a much higher affinity than oxygen, thereby reducing oxygen transport to tissues. This condition may lead to various physiological and psychological effects, particularly on the nervous system.

In the literature, CO exposure has mainly been investigated in the context of acute poisoning cases and smoking-related effects. However, studies evaluating the effects of low-level and chronic CO exposure-more commonly encountered in the general population-on pain perception, sensory functions, and psychological status are limited.

Tobacco smoke is an important source of CO, and exhaled CO levels have been shown to be significantly higher in smokers and associated with various neurocognitive changes. Evidence suggests that CO exposure may influence anxiety, depression, attention, and cognitive functions. In addition, some studies indicate that CO may affect brain regions involved in pain perception and emotional regulation.

However, there is insufficient research examining the relationship between low-level carbon monoxide exposure and pain threshold, sensory functions, anxiety, depression, and subjective health complaints within the same study framework.

This study aims to evaluate the relationship between exhaled carbon monoxide (CO) levels and pain threshold, sensory functions, anxiety, depression, and subjective health complaints in healthy adults. By examining the multidimensional effects of low-level CO exposure, the study seeks to fill an important gap in the literature and provide a more comprehensive understanding of this issue.

Carbon monoxide (CO) is a colorless, odorless, and toxic gas that can adversely affect human health through both environmental and behavioral exposure pathways. In the literature, most studies on CO exposure have focused on acute poisoning cases and smoking cessation processes. In contrast, research addressing the physiological and psychological effects of chronic low-level CO exposure, which is more commonly encountered in the general population, remains limited.

According to the World Health Organization (WHO), tobacco use causes approximately 8 million deaths annually and represents a major public health concern. CO, one of the main components of tobacco smoke, binds to hemoglobin with a much higher affinity than oxygen, impairing oxygen transport to tissues and leading to systemic hypoxia. This condition has particularly pronounced effects on the central nervous system.

Studies on the neurological and psychiatric outcomes of CO exposure have reported that cognitive impairments, anxiety, and depressive symptoms are frequently observed following acute poisoning. Significant associations have also been demonstrated between the severity of poisoning and the level of depressive and anxious symptoms. In addition, voxel-based morphometry studies have reported that CO exposure may lead to reductions in gray matter volume in brain regions associated with pain perception and emotional regulation, such as the frontal lobe, cerebellum, and periaqueductal gray matter.

In studies involving smokers, exhaled CO levels have been shown to be significantly higher compared to non-smokers and positively correlated with daily cigarette consumption. Even at low carboxyhemoglobin levels, neurological effects such as visual perception impairments, attention deficits, learning difficulties, and reduced motor performance have been reported.

At the molecular level, CO is thought to induce oxidative stress and neuroinflammatory processes, potentially leading to neuronal damage. These mechanisms may influence pain perception, sensory functions, and emotional status. However, studies simultaneously evaluating the relationship between chronic low-level CO exposure and pain threshold, sensory function, anxiety, depression, and subjective health complaints are still insufficient.

In this context, the planned study aims to systematically and comprehensively evaluate the relationships between exhaled carbon monoxide (CO) levels and pain threshold, sensory functions, anxiety, depression, and subjective health complaints in healthy adults. The study is expected to fill an important gap in the literature by shedding light on the multidimensional effects of chronic low-level CO exposure and contributing novel insights to the scientific knowledge base.

Inclusion Criteria:

• Being between the ages of 18 and 65

Having a regular smoking history for the past 6 months

No history of respiratory, cardiovascular, or neurological/psychiatric diseases

No recent surgery or trauma that could affect respiratory muscles

Not having an acute infection (especially upper or lower respiratory tract infection)

Exclusion Criteria:

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Having chronic respiratory diseases such as COPD or asthma

Having a history of cardiovascular disease

Having a diagnosed neurological or psychiatric disorder

Having a history of surgery or trauma affecting respiratory muscles within the last 3 months

Being pregnant or having a suspicion of pregnancy

This study will include healthy adult participants aged 18-65 years with a history of regular smoking for at least the past 6 months. Participants will be free of respiratory, cardiovascular, neurological, and psychiatric diseases, and will not have any acute infection or recent surgery affecting respiratory muscles.

Individual participant data will not be shared due to privacy and confidentiality considerations. All data will be analyzed and reported in an aggregated and anonymized form to ensure participant protection.