By Beth Cheshire, Lead Researcher University of Lancaster.

High-level vs Low-level

The impact of high level, short term (acute) carbon monoxide (CO) poisoning is well known and is estimated to cause around 4,000 visits to Emergency Departments annually. However, research on the effects associated with longer term (chronic) low-level CO exposure is extremely limited and the effects on the brain are currently unknown.

Case reports documenting individuals’ exposure experiences have reported neuropsychological impairments such as deficits in memory, learning and motor slowing following chronic low-level CO exposure within the home. Studies on outdoor air pollution provide further evidence of a relationship between chronic low-level CO exposure and neuropsychological effects, with reported associations between air pollution exposure, including CO, and increased dementia risk.

These findings are concerning, indicating that even small increases in CO, below the recommended World Health Organisation (WHO) guidelines, are associated with adverse health effects. This suggests that even extremely low-level CO exposure may in fact be harmful given sufficient exposure time. Furthermore, air pollution has also recently been identified as a dementia development risk factor in later life (>65).

Elderly lady washing up
Lady washing up

Risk of UK Homes

Indoor exposures present a significant concern. A number of studies have reported raised CO levels within UK homes at levels above those recommended to be safe by the WHO. High CO levels within the home are frequently caused by problems with gas appliances such as gas fires and cookers. The problem may be of particular concern within the UK with gas appliances widely used for heating and cooking. Homes in the UK are often older, potentially having older appliances such as boilers, gas fires and cookers.

The reported neuropsychological effects of chronic CO exposure present a significant public health concern, particularly when considered alongside the evidence of raised CO levels within UK homes.

Risk to Older Adults

Older adults are vulnerable to CO exposure due to increased susceptibility from reduced physiological reserve and potentially from pre-existing disease. They are also likely to spend more time at home, further increasing the risk of home exposure.

Concerns have been raised by Fire Officers who often report high levels of confusion amongst older residents. This group may be at high risk of home exposure at levels that may not be sufficient to trigger CO detectors, but nevertheless may be harmful and a possible unidentified cause of cognitive impairment.

The unnoticeable properties of CO combined with the non-specific symptoms of CO poisoning, all contribute to a difficult diagnosis. Individuals and medical professionals are often unaware of CO exposure. Consequently, prolonged exposure may continue for weeks, potentially years until CO exposure is suspected or the CO source is detected.

These exposures may therefore be responsible for significant widespread morbidity. It is this unknown risk that led the CO Research Trust to fund research in this area.

The timing of our research is opportune. Healthcare professionals are increasingly considering the accumulation of effects on a patient, using a multidisciplinary approach. Policymakers have shifted their focus onto integrated preventative care rather than an approach which reacts to acute crises.

Elderly lady

The Research Project

The research project was led by myself Beth Cheshire, as part of my PhD at Lancaster University. It had three key aims:

  • To examine the prevalence of low-level CO within older adult homes in Coventry.
  • To determine the short-term effects of low-level CO on cognitive function and mental health in older adults, who as a group are identified as specifically vulnerable.
  • To determine the longer-term impacts of chronic low-level exposure on cognitive function and health.

The Study

Working with West Midlands Fire Service, the study collected CO levels over one month in 106 older adult homes in Coventry (all below levels that would trigger a CO detector: 50 parts per million for between 60 to 90 minutes).

The residents were aged ≥58 years and were assessed on a number of cognitive functions. These included working memory, immediate and delayed memory recall, processing and psychomotor speed, visuospatial ability and areas of executive function such as the ability to control attention and ignore irrelevant information and planning and problem solving to achieve a predefined goal or solution.

The study also measured levels of anxiety and depression and collected information relating to demographic and socioeconomic factors, medical data (pre-existing physical and psychiatric diagnoses) and property information (such as home appliances and cooking and heating methods and smoking behaviour within the home). In addition to this, blood pressure and levels of CO within exhaled breath were measured. Follow-up CO monitoring and assessments were carried out at 7 months in order to examine any longer-term impacts of exposure.

The project investigated the hypotheses that chronic low-level CO exposure would be associated with impaired cognitive function and increased anxiety and depression. To our knowledge, the research is the first of its kind in the CO literature and the resulting studies provide preliminary evidence of the neuropsychological effects of chronic low-level CO exposure in an older adult sample.

Beth Cheshire with West Midlands Fire Service
Beth Cheshire with West Midlands Fire Service

The Results

Of the total 106 older adult homes in Coventry, 70 (66%) had some CO present over the initial 1-month monitoring period. Of these, 78 participants completed the follow-up at 7 months of which 47 (60%) had some CO present.

These results indicate a high prevalence of low-level CO within older adult homes in Coventry. However, the CO levels did not exceed those currently recommended to be safe by the WHO in any home.

The results from the first study suggest that chronic exposure to extremely low-level CO (≥ 1 month) may be associated with short-term improvements in performance across a range of cognitive tasks. These included areas of auditory and visual working memory, visuospatial ability, planning and problem solving, selective attention and aspects of long-term memory, including recognition.

However, the observed CO levels were extremely low (0.5-29ppm) with resulting COHb levels between 0.20-1.40%. These levels are similar to the small amounts of COHb that are in the blood from natural physiological processes combined with environmental exposure.

The results of the longitudinal study also revealed that CO exposure (≥1 month) at similar levels (0.5-25.5ppm) was associated with slightly improved function in areas including visual working memory, planning and problem solving and visuospatial ability.

However, when the longer-term impact of the first exposure and the overall total exposure (over both monitoring points) were examined, negative CO-related effects were present at 7 months in certain areas of cognitive functioning including cognitive flexibility and inhibition (the ability to adapt and respond to different task rules and inhibit previous ones).

Elderly gentleman at the door
Gentleman at the door

Furthermore, the observed positive effects of CO on memory recognition, selective attention and auditory working memory were not present at follow-up. Instead, negative CO-related effects were observed in all three of these areas of cognition. These findings suggest that the potential beneficial effects of these exposures on memory recognition, auditory working memory and selective attention are short lasting, and ultimately lead to impairments.

The results from both studies suggest that the impact of exposure to chronic low-level CO is associated with slight improvements in certain areas of cognitive function. However, the majority of these effects were short-lasting with longer-term negative impacts observed on cognitive function at 7 months.

One of the most notable findings of the study was the possible negative impact prolonged low-level CO exposure may have on age and cognition. The negative effect of advancing age on cognition was worsened by greater overall CO exposure.

This interaction between the effect of age and that of CO exposure on cognition was found in areas of executive function (cognitive flexibility and inhibition), immediate and delayed memory recall and memory recognition. This may indicate that the cognitive decline associated with ageing is potentially accelerated by low-level chronic CO exposure in these areas.

It is currently unknown why some cognitive functions are perhaps more vulnerable to low-level CO exposure, and others potentially benefit. However, CO exposure ultimately results in damage.

Further Research

If we are to advance our knowledge of the levels at which these exposures present a risk to health, we need to understand more. We need to understand the neuropsychological effects associated with chronic CO exposure at various concentrations and durations, within this vulnerable group.

Specifically, research focus upon the time point and thresholds at which harm begins and on patterns of impairment, is vital to advance understanding in this area where there is a significant knowledge gap.

Determining the levels at which low-level chronic exposures become harmful would be invaluable in informing policy, guidelines and safety technology in order to keep those most vulnerable safe
. Identifying specific cognitive areas that are affected by chronic low-level CO exposure would also provide information on possible patterns of impairment that would be invaluable in clinical settings to aid in the diagnosis of low-level CO exposure.

From a neuropsychological perspective, the possible finding of a detrimental relationship between the impacts of age and CO exposure on areas of executive function, immediate and delayed memory recall and memory recognition is most concerning. However, further research is needed in order to confirm this relationship and the nature of it. Studies investigating and identifying potential risk factors for cognitive decline and dementia development such as CO exposure are paramount.

The research makes a contribution to knowledge in the carbon monoxide literature, an area in which there is a significant knowledge gap. Specifically, it provides empirical evidence of the potential short-term and longer-term effects of chronic exposure to low-level CO, but also contributes toward identifying patterns of impairment and the thresholds at which these exposures become harmful to health in older adults.

However, there is still a long way to go before models of impairment and thresholds of harm are identified and those most vulnerable are kept safe. Further research is needed to validate these findings.

What is clear is that, with an ageing population it is important that we understand where long-term repeat low level CO exposure sits as a risk for older people amongst the range of other factors that may contribute to cognitive deficits. The novel research undertaken at Lancaster University has gone some way to beginning to pick apart these risks and to understand the effects of CO exposure on older people, but it is clear that there is more work to be done.

I was pleased to be able to present the findings of the project during the CO Research Trust's Lecture Series on Thursday 21st Oct. You can view my full presentation below.